Could be that the gearbox was incorrectly reassembled?
Jack up both front wheels, with the car in gear (engine off), turning one wheel should cause the other to rotate in the opposite direction. With car out of gear, only one wheel should rotate (both may still turn, but an assistant should be able to stop the other one easily).
With the car on the ground and an assistant, try pushing it in neutral. Now try in gear, it shouldn't move. Now try in gear with clutch depressed, it should roll.
Look to see if the clutch release lever is moving when an assistant depresses the clutch.
Only things I can think of:
Clutch assembled backwards.
Clutch not releasing.
Gearbox has two gears selected at same time (jammed).