very good, Kirse!
And there I found this one https://forums.lenovo.com/t5/ThinkPad-T400-T500-and-newer-T/... which says, literally:
Remove the battery
Press power button 10 times
Press power button down for 30 seconds
Press power button twice
Attach ac adapter
Press power button once. PC should come up. Perform normal start up. Everything should be fine.
Shut down and reinstall the battery