how can i fix my petrol mower,mountfield tuffcut,briggs&stratton ?

my mountfield"tuffcut",briggs&stratton quantum xte 55 won't start,the pull rope won' turn the engine over,engine seems seized,about an hour prior to that whilst working,engine was throwing out a lot of light blue smoke,but no abnormal noises coming from the engine.Tried for several hours but still the pull rope won't turn the engine hence failing to start.Would be very grateful if any kind person could advise me in this matter.Thanks