PC won't power on after I repasted the CPU. What should I check?

First, unplug the PC from the wall. Since the CPU came out with the cooler, check the pins on the underside of the CPU for bends or damage, and inspect the socket too. Don't force the CPU back into the socket if it doesn't drop into place with the socket lever open.

Also check that the CPU power connector, main motherboard power connector, cooler fan and any other cables disturbed during the work are firmly connected. If the pins are damaged, that could explain the problem, but it doesn't necessarily mean the whole PC is ruined.

What does won't power on mean here? No lights or fans at all, or do the fans spin but you get no picture? That's a useful distinction. Check the power supply switch and wall connection, then make sure the case power button connector is still seated on the motherboard.

If the connections and CPU look okay, you could also try clearing the CMOS using the method in your motherboard manual. Disconnect power first. It may help with a startup issue, though it won't fix physical damage.

Double check the temperature units before worrying about the paste. If the reading really was 90 degrees Fahrenheit, that's only about 32 degrees Celsius and isn't a high CPU temperature. A reading in Celsius would be a very different story.

Once the PC is working again, check the temperature under load and whether the cooler is mounted properly before deciding it needs another repaste.

For next time, thermal paste can stick the cooler to the CPU pretty firmly. If the PC is working beforehand, running it briefly can warm the paste. Shut it down, unplug it, and gently twist the cooler to loosen it rather than pulling straight up.

You generally don't need to keep repasting just because a game makes the CPU warm.