Check the ULS (SharePoint log files) if you have access to them or the EventVeiwer on the server.
In case you don't, do as follows:
In the URL add this at the back:
This will open a page with a list of current webparts on the page.
Delete the webparts and insert them again one by one untill the page fails. Then you'll know which webpart is causing the error.
If the page still throws an error while not having any webparts on it, if you have publishing features activated, it's most likely the pagelayout that the page uses that causes it.
Hope this helps :)