First I would turn on the glow plugs. Watching the interior light dim through the length of a cycle. If that proves out and engine was running well when you stopped using it.

Rings may have stuck just a little from sitting. If you used vegetable oil it is almost a certainty. They can stick even using diesel fuel though it seems.

I suspect a little quick start might get it running. Use it properly though. The best way perhaps would be to remove the air cleaner top. Have the car plugged in for an hour or so if practical. Start cranking and give the air intake with the cover off a small mist of quick start.

I would try this first with a warm block if possible and no glow plug cycle . Your really want to only introduce a mist of the quick start as it only takes a little many times. You just need something easier to ignite after a long sit on occasion.

Also remember these engines are diesels and want to turn over when cranking at a reasonable speed. Is your battery suspect at all?