Fatal Error.

The error message itself doesn't tell you what error occured but only when the error occured. The number is the unix timestamp of the time of the error. Check the server log (Apache and/or PHP error log). There will be an entry at the time of the error (and the entry will also contain the same timestamp number). This error log entry will provide more information about what's wrong. Without knowing more we can't give you any better advice.

Or if you have made any changes on your site or server just before the error occured (updating Elgg or a plugin or adding a new plugin or modifying any file) this would also give some indication of what might be wrong.