|The PSU (Power Supply Unit which is inside the system case) may have failed if nothing at all is happening and no lights are lit up. |
It may also be the fuse in the mains power plug, so change that first with a new one.
If that makes no difference, then I would suspect the PSU. Changing the PSU yourself can be dangerous if you've never tackled it before, owing to the fact that it stores electrical power which remains inside it even when the PC is unplugged.
A new PSU is not expensive but they cost more if you want greater reliability and/or a higher output wattage. Get a PC repair shop to fit a new one for you, it's safer all round than trying to do it yourself if you're not familiar with doing it, as it also involves disconnecting all the power leads from the various drives, then connecting the new power leads when the new PSU is fitted.
It makes sense to ask the repair shop to fit a more powerful PSU than your current one as original PSUs fitted at the factory tend to be only just powerful enough for the spec of the PC as it existed then. As you fit more drives and add-on cards, the original PSU may well be barely able to cope with the extra demand.