The computer restarted while Nebulb was working during the night.

Most likely it was Windows with one of its automatic updates.

Many times if you leave the computer running overnight Windows unilaterally decides that it is a good time to apply some update and automatically reboot the computer.

This has happened to me too, so now, before leaving the computer on at night with Nebulb working, I always try to verify that there is no Windows update pending to apply.