I believe it happens automatically the next time the computer is turned on.
I don’t have any firsthand experience with this as I have mine set for when I know the computer will be on, but I recall reading a post on the forum by a user that wasn’t happy to find a scan started without asking first after a missed scheduled scan.
Yes, this is embarrassing for Comodo isn’t it–their software is otherwise darn sophisticated, but it’s really stupid in a couple of practical, day-to-day ways. If your PC is off it doesn’t scan (or tell you about it) so you have to leave your PC on 24/7 or otherwise do tricks-in-the-air to get scans to run at some time when it won’t be a bother to you. Next you try Sleep mode and it doesn’t wake your PC to do a scan. And you learn quickly also that if you DO try Sleep mode, your PC goes to sleep while scanning, which resumes when you awaken it, just when you’ve turned it on because you want to use it, and not wait for last night’s scan to finish.
So Scheduled Scan doesn’t work in a practical way. I suppose in their defense the Scheduled Scan thing is only a double-check that nothing’s crept-in to your PC, cuz it should have been preventing infections all along. So we can just turn it off I suppose (not very reassuring is it).