Autobox not moving in reverse

Hi guys .can someone throw any light on this plz.i did a engine conversion on r/r classic 1988 3.5v8 .converted to 200tdi.used original box autobox was working fine before i pulled it apart. I finally completed the restore of motor and finally started motor for first time today. Now when i select drive and 123 forward gears fine .but when select reverse engine dyes but motor will not move .level is good along with new filter and oil.what do you think it could be has stoodfor 12months with no oil in .