|When you removed the components did you leave the cpu and ram in it cause it won't work without them. Otherwise you've likely confirmed the problem is not with the removed items.|
Just some general things to check:
Did anything happen to it between the last time it worked OK and the first time it didn't?
The fact that LEDs flash and fans and driives spin only means the power supply is putting out 5 and 12 volts. Since the power supply must provide other voltages at sufficient current we don't even know if it's good.
If the power supply has a 115/230 voltage switch, make sure it's in the correct position. DON'T GUESS and only switch it with the AC cord removed.
With the AC cord removed, open the case and remove and reinsert the 20-pin (or 24, whichever you have) main power supply connector to the motherboard. Do the same for the 4-pin atx connector--the one with 2 yellow and 2 black wires. That way you'll be sure they're making good connections. Then plug the cord back in and start it up.
If that's not it, it's probably either the PSU, motherboard or both. If possible swap in a known-working compatible power supply. If that's not it it's probably the motherboard.
Those were some things I've had personal experience with. There's some other suggestions here: