I think you may need to call the product helpdesk for this one. I've looked at the user manual and normally I would suggest doing a reset, but you have to do this via the handset.
The only thing you may want to give a try, is removing the batteries from the handset for a few minutes and see if when replacing them the handsets looks for the base. Let us know how you get on.
Product helpdesk details
If your phone is within warranty:
By phone: 0808 100 6556
Monday to Friday 9.00am to 5.30pm, Saturday 9am to 2pm
By email: Click here for email support
If your product is out of warranty contact BT's recommended agent
By phone: 0800 980 8999
Follow the process on page 29 of the user manual to register each handset to the new base unit. You have to treat each handset as if it was an additional one.
See paragraph 3
Its the base station menu which you use, see page 29.
You have a limited time to do that. During that period you should be able to access the menu on the handset.
If all fails, then only the helpdesk may be able to help, but I suspect that you may have to reset the base unit to factory default.