There can be a number of reasons for an Internal Server Error. Here are some thing you can do to solve it:
The most likely issue is a corrupted .htaccess file. Log in to your site root using FTP and rename your .htaccess file to .htaccess_old. Try loading your site to see if this has solved your problem. If it works, make sure to visit Settings > Permalinks and reset your permalinks. This will generate a new .htaccess file for you.
Try deactivating all of your Plugins to see if it is a Plugin
issue. If you are unable to access your WordPress Administration
Screens, deactivate your Plugins via FTP by following these instructions.
Switch the Theme to the WordPress Twenty Sixteen Theme to eliminate any Theme-related problems.