andyi on "500 error"

Hi there
I was upadating our site – by inserting html5 video code in page & css & functions files – i was the hit with erro 500 as follows:
The website encountered an error while retrieving http://www.signlanguageinteractions.com/signlang/wp-admin.php. It may be down for maintenance or configured incorrectly.
Error code: 500
I now cannot get into any pages – front facing or back end – is there any way to ‘roll back’
site is: http://www.signlanguageinteractions.com/signlang – it was near completion and was going to launch on Thursday Grrrrrrrrrr
any help appreciated
Andy