The first step I would do is to shut down the computer safely. I would then disconnect all the power cords. Next is to check both the AC and DC power connection. To ensure the power cables are seated properly, unplug them first and then plug them back much later after you have checked everything. Open the case and make sure motherboard and disk connectors are seated firmly. I would also unplug the entire PSU and use a multimeter to check for the standard voltages. If they are below the recommended, I would consider replacing the whole PSU. If the PSU is dirty, I would consider carefully removing the debris. I would also want to confirm there is proper ventilation because this may sometimes cause the power supply to fail. I would also want to test the power extensions and use another one if possible and plug the power cables right back in.