Stannah 420 Lift start/stops stuck at top?

Hi, We have a stannah 420 which is now stuck at the top of the stairs. It goes to start as normal but stops instantly as if there may be an obstruction. No obstruction that we can see. This happened a week ago as well and we moved seat, swivel and anything else and it worked again just as quick but tried the same this time and unable to get it to work. Any clu what i could now try? Thanks