"Unable to load nib file: MainMenu" I have a Mac with Yosemite (don't remember the exact version)

I used CleanMyMac today, but it broke something; after using it, I can't open most Apple applications (I can't open App Store, Mail.app, Safari, System Information; it's not working).

Opening the console, I found that the problem seems to be 


  24/01/15 17:35:36,253 Mail[468]: Unable to load nib file: MainMenu, exiting
  24/01/15 
  17:35:36,255 com.apple.xpc.launchd[1]: (com.apple.mail.7700[468]) Service exited with abnormal code: 1


This happens with almost all Apple applications.

All other apps work fine; Chrome, iTunes, Finder, etc.

I need to find a solution that does not involve bringing the computer to the Apple store; I have a lot of exams in these days and I need my Mac to study.
Just to let you know, I solved the problem by reinstalling the operating system.

This can't be done directly from the app store (because it was not working)

Instead restart the mac and when the grey screen comes press Cmd + R; then connect to the internet and reinstall it. 

I did a backup of my data beforehand but fortunately I didn't need it as all my files were in place.