window rail wobbly

Hi

The rail of one of the bath room windows has become wobbly when I try to open it wider. I found the cap of one of the bolts has fallen off, as shown in the photo. So that end of the rail is wobbly. It seems it is not fixed there by screw. Maybe because of the moisture in bathroom, the bolt is not working any more?

So is there any easy fix or I have to call in a window repairer?

Many thanks

Its the head of a pop rivet, your hinges are pop rivetted on, some are and some are screwed on, see if you can poke the remaining pop rivet through and put another one in its place, you'll obviously need a new rivet and a rivet gun, either that or find a suitable self taping screw and put that in instead