assuming disk is NTFS
(default for XP onwards and is a Journaling file system
), it isn't doing OS updates then you'll be very unlikely to have an issue with the operating system.
When the power comes back on and especially if it is erratic then hardware may fail if it is on way out - but you'd be unlucky and it would likely fail soon anyway.
I have switched running PCs off many a time - either by mistake or because it won't finish hibernating/shutting down (really must workout which app/driver kills it) and not had an issue on restart.
Nostalgia is memory with the pain removed