If your printer doesn't turn on, keeps turning off, or is stuck in a boot loop, see here how to fix the issue.
This can happen if you are trying to re-flash the printer firmware, or on rare occasions when trying to start a print. The solution for this issue is to turn off the printer on the PSU button and turn it on again (do not use the LCD reset button for this issue).
In case the issue is not fixed, make sure that all the cables are properly connected to the main board.
If your printer does not turn on at all, then there might be a more serious problem present. Try the following steps:
If your printer happens to power off during the initial preheating process, there's usually no serious damage. Most likely you just have the voltage switch set to 220V in a 110V power environment.
Every PSU has a green indicator LED to show whether it is active or not.
The silver PSU has this located inside its case, seen through the metal cover, in the bottom-left (purple arrow).
The black Delta PSU has this LED hidden behind the cover of its terminals. Remove the two screws on the cover (green circles) to reveal the terminals, for probing with a multi-meter, and the green indicator-LED (orange arrow).