Dead power supply?

It looks like the power supply on my fathers computer went out today. ( push button and nothing happens) no lights, no fans, no sounds of any kind, just a dark silence.

(It is plugged in and there is power to the outlet)

How to I tell what kind of power supply to get for it and are they difficult to replace?

If we manage to replace the supply will his files still be on the hard drive or will everything have been lost?

Are we talking hundreds of dollars for the drive or under a hundred? If it is too much I'd rather buy a new one.

Any thoughts or help? I don't have any of the specs on his system right now but am hoping you can give me some general idea of what to look for and expect.