Maybe he turned Free Agent, and wasn´t resigned??
Look at the FreeAgent Market.
Did ou edit him while he still was in the Draftpool, or after you drafted him?
Because you should wait until the Offseason is over and the New Season begins before editing him.
How to get him back:
Look for a Save where he is still in your Roster, and then copy him in your current Dynasty.
Hope tha would help.