hi i fot a astra van but locked in gear?

hi i fitted a new engine in my van but now i cant ge3t no gears and i cant move the van with the clutch in either, its like if its siezed i tested the engine before fitted i know thats good, its something to do with gearbox, it feels like if its stuck in gear, cant push it as it locks up and cant start off the key as it locks up then too, before i changed engine i had all gears, have anyone got any ideas, i changed flywheel over and clutch too as they were diffrent sizes