Happens a couple times here as well. This happens to all manual cars I've owned. Simple solution is as Colin said, shift into another gear and try again.
The gear tooth position with opposing gear tooth is blocking each other. Kinda like >< and gets jammed. Putting into another gear will shift the tooth a bit and it will slip in again.
If it happens often, you just have a very good luck on having the rare incidence of gear tooth position perfectly blocking each other. Go buy a lottery ticket.