Maybe you installed that mod incorrectly. Maybe it's not compatible. Either way, you want to play minecraft again. But how? The process is simple.
1. Type in %appdata% in your search bar.
2. Navigate to your .minecraft folder and back up all of your worlds in a separate folder in the appdata section.
3. Go back to your .minecraft folder and delete EVERYTHING in the folder. You should be left with an empty .minecraft.
4. Start minecraft. Minecraft will download all the files again, and you will have a fully working, vanilla minecraft again.

If you wish to restore minecraft from a backup, follow these instructions..(you must have first made a backup before attempting this)
1. Navigate to your .minecraft and delete all the files and folders. Again, empty .minecraft folder.
2. Go to your minecraft backup. Highlight all the files in the folder and press CTRL + C.
3. Navigate back to your .minecraft, right click the emptiness, and click paste.
4. Start minecraft. You should have a restored version running.