After I tried to install the multiple admin add-on my admin page went blank giving error 500. So after searching in the forum I found that in CPanel I can find why the error came.

The error was a file that was calling the same thing that was called in another core file. So I deleted it (plus I deleted the whole files that came with the multiple admin addon) and the login page for admin started working. And so the index of admin also is working.

But the problem is every single other pages in the admin area (orders, categories, extra pages, languages,...) they all give a blank page with this error :

HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.
here's my site just in case:

What shall I do? My cart is working fine, it is only the admin part that is not.

You need to go back and check the code on those pages. It sounds like you have code in their that crashing the web app. What you can try is if you access the page with IE, go to the advanced options in IE and clear the "Show Friendly HTTP Error Messages ". You may get some more information on the web page to help you determine what the problem is.