very good, Kirse! And there I found this one https://forums.lenovo.com/t5/ThinkPad-T400-T500-and-newer-T/Fix-for-laptop-locked-will-not-power-on/td-p/394705 ht

very good, Kirse! And there I found this one https://forums.lenovo.com/t5/ThinkPad-T400-T500-and-newer-T/Fix-for-laptop-locked-will-not-power-on/td-p/394705 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