You must be talking about the Valve Body Gasket? If you are that was a pretty complicated job, and very easy to screw up, you will probably have to take it to a transmission shop now, a shop that knows about German transmission, these are a few ideas that you could have done wrong
1. When removing the pan you could have hit one of the solenoids knocking the loom loose, and
2. If you removed the Valve Body two replaced the gasket, you may not have the loom hook up correctly, and the balls back in proper order, and
3. Upon reinstalling the Valve Body and the pan you could have pitched a wire on the loom.
My suggestion to you is to take it to a transmission shop and make sure they know about BMW 740 transmissions