No, it is actually very easy to fix!
If you confirm that flipped focusing screen is the real problem, I’ll show you how to do that yourself..
Gimmie a sec, I’ll try to find the link..
Before you start listening to someone's advice and taking your camera apart, why not call the shop that did the work and tell them your problem? For a normal sensor cleaning the focus screen is not removed and should not be installed backwards. I am not saying that this could not be the problem, but make sure this is the problem before you create another one.
I know that removing and installing the focus screen is fairly simple, but it is slightly irresponsable to start suggesting that someone take their camera apart based on a few lines of text at an online forum. I am pretty sure that Hermento's intensions are good but just try to send him the bill when you break your focus screen or its mount and see what happens. Call the people that cleaned your camera!

