HP3805 swing press stuck!

[Baldy]

Hoping someone can offer some advice as really not in a position to fork out for another press...

So, I was pressing some sub fabric last night and the press decided it would only swing open about a quarter of the way before becoming stuck solid in that position.
Even though I was only using low-med pressure I turned the pressure knob to see if lifting it would help get it moving but this just unscrewed the knob (and the nut holding it to the thread) off completely.
Has anyone else had a similar issue, or have any ideas?