How can I tell why my computer restarted?

I leave my computer running 24/7. I've noticed a few times recently when I unlock it that it restarted and I have no idea why. THis didnt used to happen I just moved from one place to another but thats the only change the setup is the same. Doesnt restart while im using it and its not too hot. I'll just use it as normal. Lock it, come back the next day and it'll have restarted at some point in the night