|I'm assuming the subject computer is a desktop computer. |
(If it's a laptop, or netbook, your computer probably cannot be fixed without spending a lot of money replacing major parts.)
Power spikes and power surges can damage anything connected to the mboard.
When they do damage something, they most frequently damage only the power supply, but there are exceptions.
If the PS is an el-cheapo model, it's a lot more likely that something else was damaged, most often the mboard. E.g. BESTEC power supplies are used in emachines desktop computers and some cheaper brand name computer models, and they're a lot more likely to damage the mboard, or other things, while failing.
If it was a lightning strike near your location, or a lightning strike on your power grid, that caused the power problem, then it's a lot more likely that more than just the PS was damaged, even if you had everything plugged into something that protects against power surges and spikes.Lightning strikes can even jump switches that are switched off, if the power cord to the computer was plugged in at the time.
The only way to tell for sure whether only the PS was damaged is to try connecting another known good PS that has enough capacity to handle your system.
If you can borrow a used PS from another computer you have, or that a friend has on their computer, and try hooking that up to your system, do that, before you take the risk of buying a new PS.
OR - take the computer to a local place that builds and repairs computers, and have them try a known good PS with your system - you may be charged a small fee.
If the original PS is failing or damaged, you can usually replace it with any decent standard sized standard ATX PS with the same capacity or greater.
Standard (PS/2) power supply size - 86mm high, 150mm wide, 140mm deep, or 3 3/8" h x 5 7/8" w x 5 1/2" d , or very close to that, though the depth can be more or less for some PSs.
Don't buy an el-cheapo PS.
See response 3 in this:
Your power supply must have at least the minimum capacity required to support a system with the graphics card you are using installed, or the max graphics card you might install in the future.
(Onboard video - video built into the mboard - IS NOT A CARD ! )
If that info is not in the ad for the video card, you can go to the video card maker's web site and look up the specs for the model - often under system requirements - the minimum PS wattage, and, more important, the minimum amperage the PS must supply at 12v is stated. If you don't find that, any card with the same video chipset including any letters after the model number has very similar minimum PS requirements. Some power supplies have two or more +12v ratings - in that case, add those ratings to determine the total +12v current capacity.
If you're a gamer...
In most if not all cases, the max capacity rating of the PS is an intermittent rating. It's recommended that you do not load your PS to any more that 80% of that rating if you are going to be using something that puts a constant load on it, such as playing a recent game for hours on end. In that case, you multiply the min capacity stated for the system with the particular video chipset on the card by 1.25 to find the min. capacity of the PS you should get.