|psu will have a label on it with wattage etc. depending on the type, a new one does not have to be identical . eg. if you have a standard ATX format PSU with say 250 watts on it then one with higher wattage will do, depending on where the label is stuck to psu you might need to remove psu from computer casing to see it.|
Some makes of PC eg. Dell, Compaq HP etc use special shape PSUs in some of their PCs and some also have different wiring plugs in which case you can only use their special PSU in those PCs.
The PSU is the most common part to fail in a PC I reckon.
A PSU outputs various voltages. If one voltage output fails then the PC will not work, but sometimes 'bits' of the PC might show some sign of life eg. lights / fans