oops, does that mean you powered the board up and measured it inside the PC ?

that wasn't necessary as all caps (of a group) are oriented identically and it's pretty obvious which connector is ground and which is supply...
anyway - here's a couple of things to check
did you apply too much solder, shortening another signal ?
when the cap came loose, was it by force or did it plop off by itself ?
in the first case (unlikely though) a part of the circuit board may be damaged
worst case is you erased the eeprom of the board (or modified it's content), by accidently hitting
that magic line.
But usually it's just a bad placement of the board in the slot - remove and plug back in or change the slot.
good luck, Tom