|"You may have to remove the cpu and reapply a coat of thermal paste. "|
He (or she) doesn't need to do that unless he (or she) was messing with installing the cpu heatsink or had unclamped the heatsink from the cpu since the computer last worked properly.
Thermal paste or compound or grease or a thermal pad doesn't suddenly go bad, and sometimes the paste or compound becomes like crazy glue and in that case it's extremely difficult to get the heat sink unstuck from the cpu. You don't try to fix what ain't broken.
I'm assuming this is a desktop computer.
The cpu heat sink must be installed properly on the cpu. If you (or anyone) have not messed with that, there's nothing wrong with that.
There must be a fan, preferably a cpu fan, on the cpu heat sink, or a fan in the computer case blowing towards the cpu heat sink (e.g. some computers have a fan and a duct to the heat sink).
There must be a female wiring connector from a fan , preferably a cpu fan, that has 3 or 4 wires, connected to the 3 or 4 pin cpu fan header on the mboard, and the fan must spin, and spin faster than a minimum rpm, otherwise most mboards will shut down the mboard in a very short time when no rpm or not enough rpm is detected from that header, to prevent the cpu from being damaged by overheating.
Sometimes the cpu fan does not spin when the power supply is failing - try the fan connected to any spare fan header on another working computer.
Failing power supplies are common and can cause your symptoms.
Check your PS.
They often partially work, fans and hard drives may spin, leds may come on, yet you may get no video and the mboard will not boot all the way.
See response 4 in this:
If it is failing, 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 minumum 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.