Good recs above. Presuming the clutch pedal hasn't disengaged the clutch with the pedal down, sounds like a leaking/blown slave or master cylinder.
+1 on @nvu's checks for fluid, and great note by @forester2002s - don't overlook the obvious.
Haven't had the slave or master go out on my stock 2010 WRX (yet), but have experienced this on other vehicles, as well as a snapped clutch cable on an older '84 GL Hatch. Definitely a bit of a confusion moment for a sec, then the ah cuhrap...
Brake fluid-y jobs are one of my least favorite just 'cause I hate the stuff (anything involving grease requires executive override on procrastination to tackle that task, though nitrile glubs make it much nicer), but neither of these is too hard to replace, as I'm sure you're aware. Definite a nuisance. Worst part, imo, is the cleanup, and getting someone to pump the pedal to bleed.