|You probably have some other problem.|
Did you remove the cpu then re-install it ?
I'm assuming you didn't. That's completely un-necessary, unless you needed to change the cpu.
The cpu WILL overheat if the cpu heat sink is not installed so that it's sitting flat on top of the cpu, but it takes time for that to happen - e.g. if it has cooled to room temp before you start the computer, the cpu will work fine until it gets too hot.
Modern mboards will automatically shut off the mboard when the cpu temp goes above a certain maximum.
If a 3 or 4 wire fan is not connected to the 3 or 4 pin cpu fan header, modern mboards will shut off the mboard in as little as a few seconds when no rpm is detected from the header.
Check to see whether there is something that you unplugged and forgot to plug back in, or a connection of something that you un-intentionally have loosened.
E.g. most mboards have TWO power connector sockets - the main 20 or 24 "pin" one, and a smaller 4 or 8 "pin" one, that must be connected to the power supply.
Some video cards installed in a slot have one or two power sockets that must be connected to the power supply with wiring connectors that fill the socket .