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Thank you for contacting the Intel community.
You can check the Blink Codes and Beep Codes for Intel® NUC here:
Is the LED is ON (solid color)?
Has anything being change on the NUC before this happen? For how long the unit worked?
If any new hardware has been recently added to the computer, remove that hardware to make sure it is not that causing the issue. If after removing the new hardware and your computer works, it's likely that the computer is either not compatible with the new hardware device or a system setting needs to be changed to work with the new hardware.
You can also try rRemoving any disk, CD's, DVD's, RAM that are in the computer and if any USB devices are connected disconnect all of them. Reboot the computer and see if the system stays on. If it stays on, start adding one at a time to see what could be causing the problem.
I hope that helps.