I'm having a rather odd issue with my Canon 1100D, whenever looking through the viewfinder to take pictures the auto focus will not work. I am unable to release the shutter, the same goes for manual focus. However when viewing a live image on the monitor the lens focuses and releases the shutter just fine and a picture is taken, again this is the same for manual focus. I've already ruled out the possibility of this being a lens issue as the same thing happens regardless of what lens I have attached. Any advice would be very much appreciated.
tl;dr: It looks like an AF system problem; servicing required.
Details: Unless in LiveView, your camera uses a separate AF sensor. Its faster and more precise than liveview one. Look closely at a mirror - notice there are two mirrors actually. Half of the light goes to the viewfinder, another one is bounced to an AF sensor. Here's a picture:
The point is: if, by some reason, one of the elements gets slightly off its place, AF fails to work. It needs to be "tuned" again.
Yeah, all of the above does not apply to the LiveView. While there, mirror flips up and focus is achieved by completely different means.