Cannot start engine after refitting transmission

check for delivery of fuel and also check for a spark if both of these are present then it should really start also check the multi plug in the corner for the engine wiring as these can some times get knocked out slightly also have you out the earth strap back on to the gearbox from the battery

has it gone a flywheel sensor mounted on the front top edge of the gearbox it may be this has been damaged during the gearbox refitting it is usually black with 1 10mm bolt or torque bolt holding it with a wiring plug on it