The clutch pedal.
it might not have any hydrolic fluid in the clutch master cylinder to engage the cluth or your clutch might me shot and just won't engage at all meaning you need a new one (was there a smell in the car right before you couldn't use clutch? that might of been the clutch burning up). don't quote me on all that I'm just telling you what you can check.
An air conditioner clutch will not engage without voltage. If it doesn't engage, the compressor will not turn.
low freon
Every time you engage the clutch, the clutch disengages from the transmission/engine. Which in term engine revs up, until you disengage the clutch.
If the clutch won't engage, the clutch disc or pressure plate has probably broke apart. If the clutch won't disengage, the clutch hydraulic fluid reservoir is empty because of a leak in the system.
Speed up to over 150 km/h. Engage reverse gear and release clutch pedal. You'll fry the clutch almost instantaneously.
Is to disengage and engage
bad clutch cable, or not enough oil on the clutch plates.
The STX-38 should have an electric clutch for the mower deck. The switch to engage it is on the right rear fender. It is a toggle switch. You will have to pull up on the switch and forward to engage the blades.
you need to bleed your clutch fluid
check the clutch fluid or maybe you need to purgue