Verified answer

It could be a mod that is the issue. If you tried to install a mod, make sure modloader (needed for many mods to work) is downloaded and (MOST IMPORTANT!!) go to your .minecraft folder. Press bin (i think) and open the minecraft.jar file using winrar or something similar. Delete the META-INF folder. That folder stops all hackers from...well, hacking. Mods will not work with that on! Also, make sure that the .minecraft folder is completely intact. Make sure no outside files (other than mods..) are put in and no files are deleted. You can regain the files by downloading them from minecraft site (I believe) or just press force update. That will re-download the essential files.

Hope it helps!

if its doing that then its the mod that happend to me just go to youre folders and delate the mod