Can anyone offer a diagnosis ?

My recently acquired Mod 5G has tonight developed an issue.

I've probably put about 100 pellets through since I purchased the pistol on BBS, the previous owner did a reseal and the pistol shot very nicely on my 6 metre cellar range.

Fast forward, tonight I put about 20 pellets down then when I came to shoot the trigger seemed ok on first stage take up then it was as if locked. Didn't want to force it obviously so re broke the barrel and the gun instantly went off.

No pellet discharge as the barrel was in the open position but not at full cock , just open pre spring cocking tension .

Re cocked , aim at target , nothing, locked trigger.

Open pistol and pow !

Same story.

Put gun down and make a brew.

Any thoughts ?