The RRoD can be caused by many things, including an emergency cut-off that trips in the event of a power surge or power cut.
To rule out that you haven't just caused this emergency cut-off to trip when you unplugged it, you should completely unplug everything, i.e. unplug the power supply from the wall, unplug the power supply from your console and unplug the two halves of the power supply.
Then once you've unplugged everything, wait for 5-10 minutes to give everything time for any residual electricity to discharge, which in theory should reset the emergency cut-off functions in both the PSU and console, then connect everything up again and it should be fine.
If you still get a RRoD after this, then there is a possibility that you've damaged the PSU (and the 360 is detecting the incorrect voltage coming from the PSU and shutting down with a RRoD to prevent further damage), so if you can, try to find an exact replacement and test that too see whether it's just the PSU that is damaged. If a different PSU works without tripping a RRoD, then you'll just have to buy a new PSU. If the different PSU still causes a RRoD, then the problem is with the console and it would need to be fixed professionally.