My thoughts is that the fan just isn't needed at the moment. The card of course has to get warm for the fan to kick in. So try playing for an hour or two on a graphic dependent game and see if it kicks on. Really this could also depend on the type of fan/card. You may also want to check the connections to see if they weren't dislodged after taking the side off or being transported. It too may very well be broken. Best of luck!
I have done to my old Ati Radeon x1650. With mine, the pc had to be on for say 30 mins (a clue) then only when I ran a graphic intensive game the problems started, (another clue.) I could hear my fan slowing down, over time, so I took the card out, and under the fan motor is a sticker which covers the fan bearing. I put in 2 drops of '3 in 1' oil, then put card back in pc. Now I can hear the fan running as it used to, and no crashes or reboots.
