Fixed unbootable PC by entering BIOS Recovery without restoring BIOS, after intermittent wake issues

I'll explain the sequence of events and then perhaps someone may have an idea what might be wrong with my XPS 15 9570.

I've been having an intermittent problem for a few months where sometimes it failed to wake from sleep mode after being left idle for at least several hours. Each time, I had to hold the power button down for a few seconds to invoke a cold boot.

Then this stopped working also. Holding the power button down for a few seconds or more (up to 35 seconds) would only result in the keyboard lights coming on and the fan whirring for a few seconds. I repeated the process numerous times. I also disconnected and reconnected the battery, but still no joy.

I then initiated BIOS Recovery, but the battery level was too low to proceed. I pulled out the power cable, then it successfully booted up.

Note that I always ensure my drivers are fully up to date using Dell Update. I have done a full diagnostics check in the BIOS and all tests were successful. And I have checked there are no viruses on my system.

What could be the cause of the above symptoms, in particular the intermittent failure to wake from sleep, given the sequence of events?