Having just changed my clutch master cylinder, there are two possibilities.
One, you have no brake/clutch fluid in the reservoir. The clutch pedal won't come back unless there is fluid.
Two, the overcenter spring above the clutch pedal is broken, or more likely the plastic guide rod inside that coil spring is broken. The part is inexpensive at the dealer. When reinstalling the overcenter spring, you have to keep it shortened. I did this by placing the spring in a vise, tightening it to reduce spring length a good inch and a half, then holding it in that position with two bits of wire installed opposite each other to hold the spring shorter. Once installed, cut the little wires and pull them out with pliers.
Refer to the appropriate diagram in any online fiche. I used realoem.com. Bentley E36 manual page 210-3 shows the overcenter spring hanging down vertically, it is installed 180 degrees from the way it is shown.
Repeat, you have to have fluid in the system (and a working clutch master cylinder) plus the overcenter spring correctly installed, to have the clutch pedal come back once depressed.