Well, belts just go bad over time. They have to start squeaking at some point, and it may just be a coincidence that it started squeaking after you were away.

Grab all the belts and tug on them a little bit to make sure they are properly tensioned. You should be able to move it a little bit, but it should feel pretty snug. If they are all nice and tight, then it may be time to replace them. Belts are cheap