Depends what she means by dead. Does the monitor show any signs of life (on light, for example) or does the PC itself seem dead (no power light or fan noise) or are both dead? It's important to establish this before you start bothering about replacement power supplies.
Screen messages referring to no signal, checking cables etc are usually generated by the monitor when there is no signal, which of course is what happens if the computer isn't working. What you do next depends on how much you want to delve into the insides of the computer. You will definitely be having to do some dismantling and experimentation. If you don't want to do that, you will need to take the computer to a repair shop. If you are happy to tackle the insides, post back here and we'll give you some guidance.
Yep. I would say that 80% of the time the PSU is he cause. That's what I meant by "some dismantling". A PSU change is well within the capabilities of anyone who is prepared to get into the insides of a computer.
Hi misters. You can't really have too many watts. The existing PSU should have a label telling you what size it is. As the computer is a year old, probably a 500W supply will be adequate. If you know the components of the computer, then you can work out an estimate of the requirements here click here. If you are buying a good quality PSU, then the estimated power requirements can be taken as a reliable guide. If you are buying a cheap supply (e.g. click here) then it's a good idea to allow a generous margin to ensure cool running.