Today the Asus screen refused to give way to a normal boot up again. It seemed to be preceded by general slowness. I removed and reinserted the battery which seems to have fixed the problem. I'm running a virus scan (Norton) and will back up important files.
When my laptop does that, it's usually because it has overheated. If heat is also the reason for your laptop's problem, you probably solved it just by powering off your machine for long enough to cool down. By contrast, running a virus scanner would probably not
help and may make things worse in the short run. (Virus scanners are rather intense on the hardware and may contribute to further overheating.)
One preventive measure I use is to place my laptop on top of two books---one on each side---making sure they don't block the fan. That way, hot air doesn't get backed up between my computer and my desk. And cold air can enter my laptop where the fan can suck it past hot components.