Check the engine compression. Was anything done prior to this occuring?

gearbox was removed to change rear crankshaft seal. It has an automatic gearbox. When the gearbox was put back, the car would not start

compression was checked and its ok

The tranmission work should not affect anything except the crank sensor but you are getting sparks, that indicates it should not be the cause.

Which engine do you have? If it is timing chain driven, the chain couldhave jumped time if the engine is turned anticlockwise.