The computer keeps shutting off unexpectedly or the computer will not boot up at all.
Check the connection thoroughly, many times the cable is not plugged in firmly. Check to see if the PSU is plugged into a power strip. If so, try bypassing a plugging directly into a wall. Same if plugged into a battery backup unit.
Look for reset button on the back of the power supply and push that in.
If either of the above doesn’t work, call the manufacturer if the computer is under warranty. If not replace the PSU and continue troubleshooting. In some cases the PSU is fine, but the Mobo isn’t getting the power from the PSU.