That has happened to me, too, and the only cure I could find was to re-download the missing skins.
A very faint memory tells that I've managed to fix such an issue by restarting the computer but that might have been something else than missing skins. It was back when there was no crap called "fast boot" so on current operating systems you'll have to keep the Shift button pressed while clicking shutdown (word says that clicking "reboot" would bypass fast boot but I'm cynical). Worth trying, though!