In my experience this is usually caused by the mechanism breaking- unfortunately there are a lot of poor quality versions of this style of waste.
You can replace the brass mechanism.
Unscrew the silver plug, this should come right off and reveal the brass mechanism below.
Unscrew the mechanism , will have flats to remove it with a spanner but sometimes this snaps the brass - in which case the remainder can be moved with a pair of grips/ mole wrench etc.
Take the mechanism to plumbers’ merchant and they’ll sell you the correct replacement. There are a couple of different designs and lengths to choose from.
If access below is easy (eg a vanity unit rather than a pedestal) it might be better to change the whole thing. But with tight access it’s much easier to just replace the brass pop up mechanism.