Hello, I was using a modified client, and noticed I couldn't open the menu, so I tried force updating, but I still can't use the menu... it opens for a split second then closes again. I don't know what to do... And it was working a week ago, nothings been changed.
First try to re-install Minecraft, then delete the whole .minecraft folder and try Force-Updating. But beware this will delete everything you have in Minecraft, so back up any saves or Texture Packs you want to keep.
Can't open menu