internal Server error ???

Help….

My site has been working fine… and now all of a sudden, i can not log-in and get the following error message:
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Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, [email protected] 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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How do i figure out how to fix this so i can once again log back in and admin the site???

thanks!!

Bob