Car sitting in my garage for last few weeks not driven. I went to drive yesterday and pushed in clutch but it didn't pop back up. I have not had any problem at all so this was a real surprise.
Since it was sitting you have air in the system. Pull the pedal up by hand and bleed the brakes and the problem should resolve itself. If you don't want to go to that effort try pumping it back and forth until it comes back to normal.
Either a return spring has broken or come loose, or a seal on the slave or master cylinder on the clutch has failed. Check for fluids leaking inside or under car which would point to the hydraulics. A visual inspection of these areas would also reveal a loose or broken spring.
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