I would say that there are three things to check.
1. That the fan is plugged in.
2. that none of the fan wires got cut when the PS2 was put back together.(they are very fine and snag easily)
3. There is a fuse that controls the fan motor. If you have a continuity meter you can check that out. I have noticed that they do not show perfect continuity, but somewhere along the lines of .5 ohms. You can find a fuse guide
here .