This is a rather short article, but I hope it will be useful to someone who is Googling their problem.
Sometimes, my Lenovo ThinkPad T480 has an empty main battery on Fedora Linux and just shuts down non-gracefullly at the moment I try to resume. I have experienced it a few times that it then is impossible to turn it on again. When connecting the power supply, the power button led blinks three times, but it won’t boot. The only way I have been able to fix it is by removing the back cover and disconnecting the RTC battery (also know as CMOS battery) and then connecting it again. The RTC battery was not even empty in my case, I have no idea why this fix works. I suspect there is some bug in the firmware of the embedded controller, but I have not been able to debug this.