There are two immediate possibilities. First, as Clive suggests above, loss of fluid or entrainment of air/moisture in the system and secondly the plastic or rubber pipe between master and slave has developed an 'embolism' and is expanding when the pedal is pressed meaning travel at the slave is reduced. (The latter can occur without visual register of it happening!) After a period of not using the car the first is perhaps more likely with a seal becoming stuck over time and then tearing when used again.
Check the level of fluid in the reservoir, and also its condition; it should be yellowy in colour and clear. If it is dark, or has crystals forming in it then it needs flushing and changing.
If the level has dropped, it will be leaking from either the master or the slave. Feel up the clutch pedal until you reach the clevis pin that holds the master cylinder fork; any fluid around here shows it is the master at fault, similarly feel around the fork or pushrod coming from the slave. NB: the rubber cap can conceal a leak until sufficient fluid escapes to fill it.
If there is no fluid loss then it is probably either contaminated fluid or a weal master to slave pipe.
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