When you connect it to the wall socket and turn on the switch, does it makes any relay phase changing sound (like "tick")?
Does it used to make such "tick" sound when it was good?
When it is connected to the wall socket with the mains, does it still don't give any output power? If yes: try to trace the plug line (from mains) to its output sockets and see what kind of circuits it is passing through.
Plug from wall socket ==> current & voltage monitoring circuit ==> output socket of the ups
You might probably find it is passing through a large resistor with a very low resistance (current & voltage monitoring circuit inside the ups). Check if that resistor is okay. Try to check all the diodes and any burnout resistors. There must be some power transistors. Set your multimeter to 'continuity' mode and check the transistor pins within each other. If any two transistor pins shows FULL 100% conductance, then its a faulty transistor.