I've had one of those stick closed. I ended up removing the trap and unscrewing the mechanism from uderneath. Fortunately it wasn't too tight, so a pair of long nose pliers shifted it.
On ours, the hole in the rubber seal around the plug was a bit oversize, allowing it to shift off centre around the plug.
One side had popped out of it's groove, jamming against the edge of the plughole.
That meant the mechanism couldn't be pushed down far enough to release and pop back up as it should.
I put a bit of LSX around the groove in the plug to hold the seal in place.
Plan B, which I didn't have to resort to, was to trim off all the bits of rubber seal I could get at with a modelling knife/surgical scalpel (a craft knife blade would probably be too thick to fit in the gap between the plug and plughole) until the plug released.
Then I would have to get a new rubber seal from somewhere
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local