D15 Transmission wont engage

I'm not much of a mechanic so I'm hoping someone can give me some ideas as what may have happened.  I have a D15 with a loader that doesn't get much use.  I used it earlier this fall but hadn't started it in a month or so until last weekend when I moved it from one shed to another where it would be easier to get at if I needed it to move snow.  Today I started it and went to move it and the transmission won't engage. The PTO doesn't work either.  It ran fine when I backed it in and parked it last week and today it won't move. I will need to get someone more mechanical to help me with it but I thought I would at least try and get some ideas as to what may be wrong.

Sounds like something let go in the clutch or maybe the throw out bearing has stuck on the outer shaft not allowing the clutch to engauge. Does it have a rear lift on the tractor? if so, does it work? If you push in the clutch pedal does it come back completely to it's normal stop position? If you push in on the clutch pedal when do you encounter resistance and is it smooth or do you feel the engine vibration or noise when you work the clutch.

When pressing foot clutch doesn't seem like as much resistance as normal.  Also doesn't seem like as much resistance as normal when letting up on foot clutch.  Clutch appears to come back up to normal position.  Tried hand clutch in hi and low range and got nothing.  No engine vibration or noise when working clutch.  Didn't think to try lowering and then raising snap coupler arms - it does have a light blade attached.  I can try seeing if it works tomorrow.  I tried engaging the PTO to see if it worked and it doesn't.  It has been unusually warm here this week in west central Illinois and it is inside a well insulated machine shed so I don't think frozen water should be an issue.

Will the loader hydraulic work  re: will the loader go up.

With the return spring on the pedal linkage the pedal should return. BUt with little to no resistance I'd say stuck throw out bearing or the pressure plate has exploded. That is unlikely but I suppose something mechanical anything is possible. I would pull the inspection cover off and use a real good light to look things over as Stray mentioned. If the cover is still in place it just snaps into the hole in the bottom of the housing.

Thanks for the suggestions of where to start looking.  I'll see if I can recruit someone more knowledgeable than me to help me check it out.  As far as the loader it operates off a front mounted pump and it is working.  Do most of these problems require splitting the tractor apart to fix?

What do you mean the " trans won't engauge" if gearshift lever won't move from netral or pto lever won't move to the engauged position then the trans has frozen water in it.

yes, I agree with others about the clutch. Front mounted pump will still work. Tractor doesnt move and pto does turn . Clutch