3
I had a similar problem - earlier today as a matter of fact. I've checked around and apparently several hosting companies have decided to not allow php.ini override values in .htaccess files any more.
I had changed upload_max_filesize to allow larger uploads and my hosting company turned off the ability to override the default value. This causes php_value statements in .htaccess files to be invalid and so it throws a 500 error.
To verify this is the problem go to your cpanel (or 'other' control panel) and check your error log. You'll see a message that says something to the effect that php_value is not allowed.
If this is the problem then all you have to do to get your site back is remove the php_value settings from your .htaccess file. Unfortunately, this also means that you can't override the default php.ini settings chosen by your hosting company any more.
* EDIT * - After my initial post I did find that, in my case, this was due to the hosting company as CGI (PHPsuexec) instead of as an Apache module. It is possible in this scenario to change "most" of the php settings by removing the php_value settings in the .htaccess file as described above and then including a php.ini in your root directory with the appropriate php setting in the php.ini (upload_max_filesize = 5M in my case).
Another potential issue when switching to PHPsuexec is that you can no longer have directories set to 777. The max. allowed is 755. If you have directories set to 777 they will also cause a 500 error. You can find a more detailed explaination from one of the hosting companies
here.