It could simply be frozen up with corrosion, gunk, or something similar. This happens to be a common problem on old horizontal mills with round bar over arms. Getting those arms to move again, can take a very long time of soaking and working the metal.
I'd get a really good penetrating oil, soak it for a couple of days and then try to rotate it carefully. Just modest back and forth torque. This assumes that the shaft itself is not free, depending upon the half nut design you could have something seized up there too.
Probably more important a machine 25 years old likely could use a tear down and a clean and lube. A tear down gives you the opportunity to take a closer look at the mechanical of the entire apron.
GENTLE heat enough so you could still handle it. an oven being the ideal choice, to soften any gunk its amazing how hard old grease can get but only do this if no plastic or seals are there, unless as a last resort. The problem with old grease it is a very good seal and stops any releasing agent getting very far.
On some lathes there is kind of a lock mechanism that prevents the threading lever from being engaged while in power feed mode & vice-versa. There maybe be some cam-like levers & retention spring in the apron area. Of course your lathe may be completely different but probably worth pulling out the parts manual for a look-see. It still may be an issue of dried gunk, but might also be other things. My mechanism was somewhat sticky but with the apron completely removed I was able to cycle the action & more clearly see what was going on.
The penetrating oil didn't work, Kroil is good stuff.
There is no movement at all and no rust, I'm going to have to dig deeper like you said.
Opened the back elec. box one of 3 cap. was laying down, fixed that, machined a rectangle hole in the cover added a single outlet for a light.
They sure use small wire for the motor, 16 or 18 ga., I put a 12ga. 12' long power cord on.
2 ground wires were loose, switch was ready to fall down, tighten.
The oil in the case looks like old gear oil, I have the top off and made a gasket.
Denn, Only just noticed this post, did you get the machine sorted? Peter above is correct regarding the feed/screw cutting lock system and this sounds to me to be the most likely cause of the problem so a strip down of the apron will be needed.
Do you have a manual? I have the original that came with my machine but also look at groupsio Log In and the Emco threads, pretty sure there is a manual in the files also lots of info therein. John