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Hi! I was playing Minecraft with my Sister and I went off and on after a few hours but it Crashes whenever I open that world or any I have.

Crash log:

---- Minecraft Crash Report ----
// Why is it breaking

Time: 11/01/17 12:13 AM
Description: Updating screen events

java.lang.RuntimeException: Failed to check session lock, aborting
at net.minecraft.world.storage.SaveHandler.func_75766_h(SaveHandler.java:64)
at net.minecraft.world.storage.SaveHandler.<init>(SaveHandler.java:42)
at net.minecraft.world.chunk.storage.AnvilSaveHandler.<init>(AnvilSaveHandler.java:16)
at net.minecraft.world.chunk.storage.AnvilSaveConverter.func_75804_a(SourceFile:78)
at net.minecraft.client.Minecraft.func_71371_a(Minecraft.java:2162)
at net.minecraftforge.fml.client.FMLClientHandler.tryLoadExistingWorld(FMLClientHandler.java:695)
at net.minecraft.client.gui.GuiSelectWorld.func_146615_e(GuiSelectWorld.java:189)
at net.minecraft.client.gui.GuiSelectWorld$List.func_148144_a(GuiSelectWorld.java:261)
at net.minecraft.client.gui.GuiSlot.func_178039_p(GuiSlot.java:295)
at net.minecraft.client.gui.GuiSelectWorld.func_146274_d(GuiSelectWorld.java:77)
at net.minecraft.client.gui.GuiScreen.func_146269_k(GuiScreen.java:523)
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1674)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1024)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:349)
at net.minecraft.client.main.Main.main(SourceFile:124)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Stacktrace:
at net.minecraft.world.storage.SaveHandler.func_75766_h(SaveHandler.java:64)
at net.minecraft.world.storage.SaveHandler.<init>(SaveHandler.java:42)
at net.minecraft.world.chunk.storage.AnvilSaveHandler.<init>(AnvilSaveHandler.java:16)
at net.minecraft.world.chunk.storage.AnvilSaveConverter.func_75804_a(SourceFile:78)
at net.minecraft.client.Minecraft.func_71371_a(Minecraft.java:2162)
at net.minecraftforge.fml.client.FMLClientHandler.tryLoadExistingWorld(FMLClientHandler.java:695)
at net.minecraft.client.gui.GuiSelectWorld.func_146615_e(GuiSelectWorld.java:189)
at net.minecraft.client.gui.GuiSelectWorld$List.func_148144_a(GuiSelectWorld.java:261)
at net.minecraft.client.gui.GuiSlot.func_178039_p(GuiSlot.java:295)
at net.minecraft.client.gui.GuiSelectWorld.func_146274_d(GuiSelectWorld.java:77)
at net.minecraft.client.gui.GuiScreen.func_146269_k(GuiScreen.java:523)

-- Affected screen --
Details:
Screen name: net.minecraft.client.gui.GuiMainMenu
Stacktrace:
at net.minecraft.client.Minecraft.func_71407_l(Minecraft.java:1674)
at net.minecraft.client.Minecraft.func_71411_J(Minecraft.java:1024)
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:349)
at net.minecraft.client.main.Main.main(SourceFile:124)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at net.minecraft.launchwrapper.Launch.launch(Launch.java:135)
at net.minecraft.launchwrapper.Launch.main(Launch.java:28)

-- System Details --
Details:
Minecraft Version: 1.8.9
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 328152504 bytes (312 MB) / 544956416 bytes (519 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
FML: MCP 9.19 Powered by Forge 11.15.0.1684 5 mods loaded, 5 mods active
States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored
UCHIJA mcp{9.18} [Minecraft Coder Pack] (minecraft.jar)
UCHIJA FML{8.0.99.99} [Forge Mod Loader] (forge-1.8.9-11.15.0.1684.jar)
UCHIJA Forge{11.15.0.1684} [Minecraft Forge] (forge-1.8.9-11.15.0.1684.jar)
UCHIJA cfm{4.0.1} [MrCrayfish's Furniture Mod] (cfm-4.0.1-mc1.8.9.jar)
UCHIJA Lantern{1.0} [Lanterns and Flashlights Mod] (LanternMod-1.8.9-1.0.jar)
Loaded coremods (and transformers):
GL info: ' Vendor: 'NVIDIA Corporation' Version: '4.5.0 NVIDIA 369.09' Renderer: 'GeForce GTX 560/PCIe/SSE2'
Launched Version: 1.8.9-forge1.8.9-11.15.0.1684
LWJGL: 2.9.4
OpenGL: GeForce GTX 560/PCIe/SSE2 GL version 4.5.0 NVIDIA 369.09, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because OpenGL 3.0 is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: Yes
Is Modded: Definitely; Client brand changed to 'fml,forge'
Type: Client (map_client.txt)
Resource Packs: Resource Pack
Current Language: English (US)
Profiler Position: N/A (disabled)
CPU: 8x Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz

I tried re installing forge but it says writing protected or something.

I reinstalled Minecraft and still nothing.

Quote from MCOfficer»

delete your session.lock file.

see http://hopper.minecraft.net/help/failed-check-session-lock/ for more info.