Hello,
I would consult with yoir orthodontist. In some cases depending on prescisely how much shifting has occurred, it would be possible to return to wearing the retainers and return the teeth back to their straightened state.
As you have mentioned that you have been without your retainers for nine months, it is likely that you would require braces once again. The good news is that in most cases, the treatment is not as invasive and the amount of time you would need to wear them would not be so long.
Best thing is make an appointment with your orthodontist to determine what can be done in your particular situation.