500 Internal Server Error

What causes the following error, and how can it be fixed?

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, driver@pair.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 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
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My server of the site is FreeBSD, and

server of the site is FreeBSD, and we just move onto a dedicated server because we thought the error was caused due to traffic on our site. (it was on a shared server before). On both the shared server and the dedicated server, we keep getting the error

Any number of things could have caused the error, did you check httpd.conf for errors? Did you check your error_log for details? use '/usr/local/apache/bin/apachectl configtest' to check your config file. Was the page causing the error in plain html? or was it using some extra features? To fix the 404 Not Found error document try 'cat /usr/local/apache/conf/httpd.conf | grep ErrorDocument'
if you see something like
ErrorDocument 500 /somefilename.html
then check to see if that file exists (most likly it dosent) so either create it or comment the line out. I always compile from source with a prefix of /usr/local/apache, but your path is probably different so change it to suit you.