I got an Internal Server Error 500 while installing an addon, and now I cannot access my addon area at all. Half my admin pages give me blank pages and the main admin panel page tells me I may have outdated templates. If I try to edit/revert the templates in question it gives me a blank page.
Visit /install and click the button to rebuild the master data. That may fix the problem. If the rebuild fails or stalls then just refresh it until it completes.
A 500 error is a generic server error that can be caused by bad file / folder permissions on the server. Folders are normally 755 while files are normally 644. The data and internal_data directories are usually 777.
Also, a blank page is often a suppressed error. Try adding this line to your library/config.php file to see if that reveals any specific error message on the blank page: