Change your minecraft.jar and retry logging on. (DO NOT DELETE IT)
Find minecraft.jar in Windows
1.Click Start Menu (lower left corner)
2. Then go to Run
3. Type in %appdata%
4. There will be a window popup, browse in there you should find a .minecraft folder
5. Go inside .minecraft folder, then go into bin folder
6. Now you should see minecraft.jar
7. Change it to minecraft_2.jar and log on
Find minecraft.jar in Mac
1. Open Finder.app
2. Click “Go” in menubar
3. Then choose “Go to folder…”
4. Type in “~/Library/Application Support/Minecraft/bin” (without quotes)
5. Change it to minecraft_2.jar and log on