this happened to me a couple of times if u look under the seat there is a large metal pin that slides into the sliding mechanisim, this usually comes loose and falls out, if u see it out slightly this probably is the problem just simply give it a good wiggle and a push it will go back in
there is one pin either side of the seat
From the Front of the seat
From The back of the seat
what u may also find easier if u remove the plastic covers along side the seat u will see the machanism and how it works sorry no pics of this