How to Fix 500 Internal Error in Magento

Server: 1and1 hosting
After fixing the database issue, I found myself trap with:

1.) front end - I got 500 Internal Error
2.) back end - out of memory error

To keep it short here are my solutions:

1 & 2.) I remember doing these things:
a.) rename your sample php.ini.sample to php.ini and add these lines at the bottom
extension=mcrypt.so
extension=pdo.so
Then upload to your server in the magento's root installation directory
b.) modify the .htaccess file such that:
i.) find this line: ## this line is specific for 1and1 hosting
ii.) uncomment the lines inside
c.) search for the line: RewriteBase /magento/ and change to RewriteBase /

That do the trick for me.