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

URGENT! When going to the appointment page, this is what is shown: Internal Server Error

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

Please contact the server administrator, webmaster@jblmsantascastle.org 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.
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All other pages work fine. No plugins have been added, no changes made except to add a custom field yesterday (without apostrophes). HELP! Tomorrow is the last day to take appointments with this nonprofit!