My XPS 15 9520 won't boot after a busy day

I've got an XPS 15 9520 that's in daily use doing pretty intensive work. I've had it over a year.

Yesterday I'd left it running a task that was maxing out the memory (64GB) and I left it going overnight.

This morning it was hung - there was a Windows login screen but it wouldn't respond so I shut down by holding the power button down. When I tried to boot again it just hung on the Dell logo for hours, doing nothing.

Eventually it went to the repair download screen, I went through that and it seemed to be busy, then said it had finished successfully (no further details). No change. I did the thorough test from the F12 option, which all completed with no issues. No change.

Has anyone got any hints? I'm a bit stuck by the lack of any error messages or warnings.