Windows boot and recovery mode

Dell Inspiron 3891 about 2 years old. Windows 11

Noticed that it had rebooted a couple of times recently when I was away from the computer.

Now it wont reboot at all.

Normal boot just gives dell logo and the swirling circle.

A couple of times I have managed to get it to try and auto repair - it gets to a certain stage and then just says there is a problem and tries to reboot normally.

Got into the bit that checks hardware and it says hardware is all fine.

Done the f12 thing to get into Dell support assistOS (I think its called) It will get to the stage of downloading and says setting up the environment and signature is verified but then will go no further ( I've left it there for hours)

Stuck with what to do next