2002 Mini Cooper Cannot switch on the car

Sirs,

My Cooper since one month the gearbox was broken, I changed the whole gearbox and still at the garage since we put the replaced gear box the car is notbswitching on eventhe battery is full. I had no other problem only the gearbox.
Is that because we removed the battery cable for this period or it's an alarm issue and needs to be reset.
My mechanical is trying many scenarios with no success.

Can anybody help pls do we need to reset the computer of e car

Thanks