500 Internal Error???

[Calystos]

Hi, don't know why but for the past 15-20 hours or so I've been getting nothing but the following whenever I try to access ANY part of my website (other than cPanel of course that works fine as does FTP).

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, webmaster@pnx.x10hosting.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
Apache/2.2.9 (Unix) mod_ssl/2.2.9 OpenSSL/0.9.8b mod_mono/1.9 mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at pnx.x10hosting.com Port 80

Anyone else getting anything like this? If so anyone have any ideas how to fix this problem? As I've not updated any files and everything was working great yesterday.

Thanks loads.

Edit: Before I posted this I did check the other 500 Internal Error posts, and checked my .htaccess file but nothing appears to be wrong on that end (specially since it was working yesterday and like I said I've not changed any files since then).

Edit: Seems that my .htaccess file was to blame, but why was it working yesterday but not today I don't know. I removed the file and it started working. But the only thing I had to add a few weeks ago to it was this:

# Turn off mod_security filtering.
SecFilterEngine Off
SecFilterScanPOST Off

As some parts of my forum weren't working at all without it. I couldn't upload certain items and was getting error codes instead. And after looking up information on the official sites about it they said to add those lines and it worked. Then last night it stopped for no reason. Any ideas?