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I'm having problems trying to spawn in a world on Minecraft. Everything works fine until I spawn in one. After that it freezes with a black screen with only the Hotbar showing and crashes. Saying that there is a problem. There could be a problem with Java or some kind of script errors in a log. Not entirely sure. The crash log repeats three more times and is an error. It happened every time I tried to get in a world.

The log:

Exception reading C:\Users\Gabriel\AppData\Roaming\.minecraft\saves\UltraCrusher300's New World\level.dat

java.util.zip.ZipException: Not in GZIP format
at java.util.zip.GZIPInputStream.readHeader(GZIPInputStream.java:165)
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:79)
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:91)
at ed.a(SourceFile:24)
at bbu.a(SourceFile:83)
at bbu.c(SourceFile:67)
at bbr.b(SourceFile:58)
at bkc.e(SourceFile:36)
at bkc.<init>(SourceFile:29)
at bka.b(SourceFile:42)
at bho.a(SourceFile:367)
at bes.a(SourceFile:902)
at bhv.a(SourceFile:238)
at bho.a(SourceFile:341)
at bhv.a(SourceFile:502)
at bho.k(SourceFile:404)
at bho.p(SourceFile:381)
at bes.t(SourceFile:1586)
at bes.av(SourceFile:970)
at bes.a(SourceFile:398)
at net.minecraft.client.main.Main.main(SourceFile:124)

level.dat is corrupted, use the backup one or try to repair it

After copying the URL, do I download the text or not? Just to make sure.

no

Use what backup, and how exactly do I repair it? I looked at the files and have nothing on it at all in level.dat files

Mcedit can repair it IIRC, but your best bet is replacing it by level.dat.bak

I have three of them, I change all of them with level.dat.bak also?

only replace level.dat with level.dat.bak

Turns out level.dat was in all of my worlds and changed them. But now there is this: It could also be level.dat_old

Exception reading C:\Users\Gabriel\AppData\Roaming\.minecraft\saves\UltraCrusher300's New World\level.dat_old
java.util.zip.ZipException: Not in GZIP format
at java.util.zip.GZIPInputStream.readHeader(GZIPInputStream.java:165)
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:79)
at java.util.zip.GZIPInputStream.<init>(GZIPInputStream.java:91)
at ed.a(SourceFile:24)
at bbu.a(SourceFile:83)
at bbu.c(SourceFile:75)
at bbr.b(SourceFile:58)
at bkc.e(SourceFile:36)
at bkc.<init>(SourceFile:29)
at bka.b(SourceFile:42)
at bho.a(SourceFile:367)
at bes.a(SourceFile:902)
at bhv.a(SourceFile:238)
at bho.a(SourceFile:341)
at bhv.a(SourceFile:502)
at bho.k(SourceFile:404)
at bho.p(SourceFile:381)
at bes.t(SourceFile:1586)
at bes.av(SourceFile:970)
at bes.a(SourceFile:398)
at net.minecraft.client.main.Main.main(SourceFile:124)

The .dat_old is also corrupted looks like. Your best bet in taht case is generating a new world with the same seed and copying it to your old world, then using nbtedit or similar to copy back your inventory

NBTExplorer could also work? Or does it have to be NBTEdit? By the way, copying the new seed over to the old world has to be in Minecraft application itself?

NBTExplorer also works

You have to generate a new world with the same seed, so you set the seed of your new world to the one of your old one when generating it

How do I copy back my inventory and use NBTExplorer?

Find a tutorial for it