Toilet seat is loose?

Hi, our toilet seat is loose. The 'hinges' of the seat and cover appear to be connected to the toilet via a long plastic screw which appears to go through a nut on the underside of the toilet and up through the hinge of the seat and the cover. What seems to have happened is that the nut on the right side has come off, taking it's screw with it, so that the seat & cover effectively now pivot just off the left hinge. Is there an easy way of getting the screw and nut back on the right side? How could I reconnect the nut to the underside of the toilet - I can't see how it was fixed to it before (the left side that is still working is up against the bath so I can't get under it to see how that is fitted). Sorry don't have any make details