This morning, when I tried to turn my Lenovo P51s ThinkPad laptop on (it was on my desk, untouched from where I'd left it last night), it started the process (keyboard lights up, fan turns on), then turned off after ~2s and repeated the process ad infinitum. Pressing the power button turned it off.
This happened even if I disconnected the power supply and (!) the battery. Looks like it has some sort of internal battery, in addition to the removable one, today I learned.
Activating the hard-reset button on the back with a pin also didn't fix the issue.
Has anyone encountered something like this before?