Recently I have got a problem. My NUC7 refused to boot OS. The sympthom was exact as in this posting. The power button turned to solid amber/orange color (no blinking).
I've tried holding the power button for any length between 3 to 15 seconds, no changes.
Fortunately I solved the problem following an advice from Intel NUC community. The link to possible solutions is here.
I diassembled the NUC7 to reach an opposite side of the main board (with cooling fan and CMOS battery). I disconnected the battery for 15 minutes, and then connected it again. It solved the problem with power indicator and booting.
I just bought a NUC13 Core i5 in 2023 and instead of orange light, it just showed a white light momentarily and the fans kept running but the power led goes off within a few seconds and nothing is shown on the screen, and I just did this and it worked! Thanks a lot for this tips!