Clutch slave cylinder (inside the transmission) has failed (my synopsis), or (not likely) all your shift forks have broken simultaneously (which I don't think would allow you to shift gears, and would have resulted in a horrific noise/calamity of occurrences).
Did you notice a burning smell (brake fluid on the hot chutch type smell) after it happened?
If you have an inclined driveway, put the car in gear (with it off, don't want you running into anything) and see if it will hold. If you can put it into gear and it holds then your shift fork(s) is/are good.
The more information you can give us on what you've done, the more information we can give you on what else to look for/at...