Mine started doing the same today.
In celsius it should be reading 381 which it has done so for 5 or 6 sessions in the last 2 weeks. Now it reads in the 260's. If I remove the thermocouple it drops towards ambient. It was in the 60s when I had to leave it to return to work.
I suspect the wiring near the thermocouple has shorted. I've been using the one that came with the ebay kit which is the short screw in probe while my longer probe versions are in the mail.
I have been submersing it under the lead from the top and I suspect the wiring under the sheath has given out. I hope so anyway, that's the easiest fix.