The HP support site has the following solution for your problem. Appearantly, If turning the power off by pressing the power button and then turning the power back on, by pushing the power button does not solve the problem, the printer needs to be examined by a service technician.
When this type of problem happens to me, I will always unplug and remove all power for the device, be it a printer or computer and let it sit for a few minutes before plugging it back in and turning it back on. This gives the unit time to cool down and reset itself.
I pasted the solution page excerp from the HP support site below to show you exactly what HP suggests. I hope that you found this information helpful. If I can be of further service to you, please let me know.
All lights on the all-in-one are flashing and there is a Power Reset
error message in the LCD. You may be experiencing all or one of the following symptoms:
All lights flash and the Power Reset message displays on the LCD when the unit is first turned on or during any normal operation of the product such as printing, changing cartridges, copying, faxing, or remaining idle, while the unit is turned on.
Follow the steps below until the issue is resolved. All of the following steps might not be necessary to resolve the issue.
Step one - Turn the unit off and on
Press the Power button to turn the unit off.
Press the Power button to turn the unit back on.
Try to print a simple print job or copy.
Step two - Service the product
If you have completed all of the steps listed above and are still experiencing a problem, service your product.
If the issue is resolved, you do not need to continue with the rest of the instructions on this page.
If the issue is not resolved, continue to the next step.