"no corresponding tunnel exists"

Difficult to be explicit about this, but would appreciate any guidance.
My Site-to-site IPSEC VPN between uk office and branch (in France) no longer shows a VPN connection on the home SBG3300 status window. Connection (and Ping) does not function though Fr branch ZyWall shows caption above (receive IPSec ; no tunnel..) . The Zywall shows everything working, and accesses uk servers without problems, but there is no reciprocal connection.
The setup  has previously worked reciprocally for  a few years, or more..I have 'nailedup' and DPD on both devices.
Any previous experiences or suggestions most welcome!
Thanks Jeff Purcell