The only things that come to mind is perhaps a firmware upgrade being pushed out if modem supplied by ISP (Power light flashes).
The obvious one is the modem LED lights on/off switch has been toggled.
Personally I'd soft reboot the modem/router and see what happens, particularly if something has caused the LED's to incorrectly switch off.
The only other thing that comes to mind and this is unlikely although I have seen it on equipment I've worked on and that is a crack on the PCB or a dry joint. (Usually all Led's would be out and not indicate at all)
Edited by Vorlon (Mon 28-May-18 22:48:54)