I have a 50 ft. deep 4 in. cased well with a 3/4 hp. Well pump, a 1 in. plastic water line down the well that is used for a sprinkler system. Have not used for two years, now cannot get pump primed. Worked fine before. Could my problem be inside the pump, rust or other debris?

Just to be clear, when priming the pump you are probably unscrewing the plug (or pressure gauge) atop the impeller cage, pouring in water until it's full, and then replacing the plug. When you fill the cage, it should hold its level once you get it full. Does it do that, or does the level drop?

I think the likelihood is a leaking footvalve that prevents the pump from holding prime. Could also be a leaking wellpipe. All of that is based on my guess that, when you fill the pump while priming, it will not stay full.

I go with a foot valve too. I question the effectiveness of a single pipe pulling 50 ft. Is that the depth of the foot valve?

Yeah. It won't pull much past 25 feet. I can only assume that though the well is 50 feet deep, the water table is much less than that.