Help wordpress update broke my .htaccess

I realise this is not theme related but could anyone advise me how to fix this?

During the wordpress update I got a white screen of death. Now with the htaccess file enabled both my sites show a '500 Internal Server Error'.

My host provider has renamed the htaccess to file htaccess.txt and now the sites show but the root sites navigation (the one I tried to update) just returns a 'file not found' and both sites much slower to load.

I have my permalinks set to 'post name'

I don't have the knowledge to fix the file and I can't simply start over because I have a geocraft theme in a sub directory that I would break?

Any help or advice would be greatly recieved.

 

Sorry I panicked a little.
All fixed now by replacing the file with one from a back up!