The bank that you received your mortgage from gets the share certificate and gives it to you only after the lein is completely satisfied. In the event that you did misplace this certificate, the board that manages your co-op should be able to give you a duplicate share certificate. Good luck with your sale.
If there is any way that I can help, please do not hesitate to contact me...
Halstead Property LLC
Tel: (212) 317-7844