For anyone coming to this thread from a Google search, my solution to this was:
Connect the D5 to a PC via the USB cable.
Open the DM5 software (make sure SuuntoLink is off)
Select the watch and go into the General settings.
Change the brightness to any other setting.
Synchronize the settings.
This will cause the D5 to restart, and after that I could access the Settings as per normal.