this can have various causes
Hair has become too long:
The bow nut is broken:
The screw has come loose from the leg:

Hair has become too long:
Hairs give way over time and then become longer and longer.At some point, the rod box comes to an end and the bow can no longer be tensioned to the required tension. This can be seen, for example, if the distance between the frog and the thumb leather suddenly becomes very large or if you unscrew the screw completely and remove the frog and hold it next to the rod. Then you can see whether the bow screw is already at the stop or whether there is still room to tighten it.
Sometimes a cover that is too long can be shortened, or a new cover can be made.

The bow nut is broken:
the bow nut is made of brass, a relatively soft metal, and the thread wears out over time and then no longer grips. In this case, the nut must be replaced.

The screw has come loose from the leg:
This can happen for various reasons, e.g. if the hole that holds the screw is drilled too deep. In this case, the hole must be plugged and re-drilled.
In all cases you cannot do anything yourself, but should contact your nearest bow maker or violin maker.