Astra MK4 1.6 16V 2001 About 70K miles

After a lot of stop start in a jam, the clutch stuck on - couldn't get out of first gear. It started earlier as 1st gear became difficult to engage and disengage and finally the clutch seemed stuck on - managed to roll to the side. After stopping, within a few minutes pumped the pedal a couple of times and hey presto clutch worked fine - drove home no problem !!
Hydraulic fluid seems OK and I can't see any leaks.

What is the likely cause ? and fix ?