After replacing the empty CO2 tank with a new/full CO2 tank, my beer is still flat. After 48 hours, it flows from the kegerator, but is as flat as could be. Is the keg ruined?
The keg should be fine. Oxygen shouldn't have gotten in, unless it was a corny keg with really bad seals. Carbonation time for a standing keg is something like a week (or more?), but I would expect some carbonation after two days. If crank up the pressure up to 20 or 30psi and shake the keg for half an hour, you can get it done much quicker.
If you don't hear CO2 going through the regulator while shaking the keg, then something is wrong mechanically. Also, if there are CO2 leaks you'll find them quickly this way.