The PSU's can go bad but they are pretty stout. I had an on/off switch break off when I dropped one from 10 feet. I bent and jammed a paperclip in the remaining switch to short the pins to turn it back on again.

One of those green lights tells you the AC power is on, the other will tell you when DC power is there, and that will only happen if the power supply is good and the switch on the back of the board is flipped on.

There are two orange lights for a voltage error and a fan error, I have never seen any of these lights activate yet.  If both AC/DC power lights are on but fans are not spinning I would say the fans are dead somehow. There is a part number for just the fan replacement but the power supplies are already so inexpensive its probably the same cost just to get another power supply.