So you have been messing with your htaccess file or downloaded a plugin for custom error pages, or messed with your permalink settings and now you are getting a 500 internal server error..ooops!
So, after much googling, and seducing the web, I came to the conclusion that, (and this worked perfectly)
Go to: Settings>>Permalinks
Change permalinks setting back to 'Defualt' CLICK SAVE
then, look at your site, Should work fine,no more 500 Error
now go back and change the Permalinks to the desired setting,
or the way you had it before the 500 error accured.
You are in business.. Time to Rock..and, or Roll :)