You're still sort of on course, but the board is pointing about 45 degrees closer to the wind, and blasting along sideways. You're still on a plane, but you're not sure you should be.
Whoa. Now that you've stopped and jumped in the water and turned your board over, you can see that the fin is still there, and just fine. (Or not, in which case you need to look for the "busted fin" FAQ.) What happened?
You've just experienced SPINOUT.
That nice, smooth flow of water on either side of the fin that was providing the (sideways) lift to keep you on course, and heading a little upwind is not so nice and smooth when this happens. Your fin has "stalled," and at the new angle of attack, 45 degrees or so, there's hardly any lift, but plenty of drag. What's more, it's a long way back to that smooth flow!
There are three causes of spinout, and even more ways to cure it. The actor in the scene described above has come to be known as "ventilation" and is the way most of us first encounter spinout; after going airborne off a wave, we lose it on re-entry. If air can get at the root of the fin, it can be drawn into the low-pressure flow on the windward side and lead to a stall. The turbulence resulting from chop and the landing helps this happen, as does putting the fin back in the water at the wrong angle - it needs to be pretty close to "on course."
The other term that gets tossed around is "cavitation," which is low pressure boiling. It's a known and well-studied problem for propellors, and for hydrofoils at speeds above 40-45 knots. You probably weren't going that fast, were you? Whatever the threshold, any nicks, dings and other surface imperfections will bring it closer, and increase speed-robbing drag as well.
The third cause is a fin that doesn't match the conditions. Too small a fin for that big sail will do you in, as will too big a fin when the wind is blowing "small sails." (The latter problem is from loss of control as the vertical component of lift from your fin wants to make your board fly.)
How do you keep it from happening? Keeping those dings tuned out of your fin is certainly important, and better fairing (smoothing of the transition) between the fin and the board can help. Technique has a lot to do with it, too. Will Estes (firstname.lastname@example.org) suggests the fix I use:
As you get better, you will find yourself becoming very familiar with the sensation of the fin just as it is about to begin to spin out. There is a moment before the spin out when you can feel the fin lose force against the water. If you act quickly to remove weight from the fin just at the moment when the force of the fin is lost, the spinout will correct before it even happens.
The side force that you apply to your board and fin is what "drives" the fin. If you drive it too hard, or when it's not completely in the water (or if you don't have enough fin to start with) it stalls. In the worst case, you have to get out of the straps and move your weight closer to the mast to get it off the fin.
Paul Billings (email@example.com) describes a more agressive technique:
There are plenty of experiments in fin design going on, and you can certainly join in that fun. Hydrofoil designers have been working around cavitation for more than 20 years, and with the sailing speed record topping 50 knots, it's certainly a problem for windsurfing's "leading edge."
If your designer hasn't made any gross mistakes (or found the Holy Grail), fin SIZE is the key parameter you need to pay attention to. If you can control your sail in the conditions you're in, but you spinout easily, or can't point as high as you want to, you need MORE fin. If the board is getting squirrely, trying to fly on its own, leaving you overpowered and out of control, you need LESS fin (and maybe less board, too).