Site won't load. Unexpected Error **SOLVED**

I've done absolutely nothing to this site for about a year, and all of a sudden I noticed that the site doesn't load, and I get an error message 'An unexpected error has occured' & 'could not find driver'. Any ideas of what could be going on?

EDIT - Apparently my host automatically upgraded my PHP version to 5.6 beta. Reverted it back to 5.4 and everything works great.

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TinyGiant replied on at Permalink Reply
This also happened to me. How did you have it reverted?
Kurtopsy replied on at Permalink Reply
I wrote how to fix it on the thread you started.