After disengaging clutch to start car, it will not re-engage

Yes I can, without issue, and the clutch pedal seems to have less resistance than before. Although, I only had driven the truck for fifty miles or so before this happened so I may be wrong in this regard. I cannot hear any noise coming from the transmission/clutch now, nor did I hear or feel anything unusual when I last disengaged the clutch the last time it was working properly. However, before this happened I was starting to notice a tiny bit of clutch slippage in fourth or fifth. This truck also had sat for about four years without being run.

Well rats. I was hoping that the master cylinder was not releasing. I have been looking at various videos for this repair, have a friend with a transmission jack, and will try to get it done at the house with his assistance. Does not look too complicated with the aid of a good jack, just a bit of muscling around. Thanks Roy!