Sounds like power isn't getting to the display, battery may have sleep mode or not inserted correctly.
The below is taken from the manusl.
5.9. Error indications and possible solutions
The system cannot be switched on(no indication on the sMMI display)• Has the battery been correctly inserted into its holder?• Have all plugs been correctly connected?• Are there any deposits (such as metal filings) on the magneticbattery plug? Please check this very carefully!• Is the battery "awake"? After 48 hours without being used, thebattery goes into a "deep sleep" and must be reactivated bypressing the battery button once.
• Has the sMMI lock been activated by the specialist dealer? If so,the sMMI only operates with the designated motor (see section3.2.14 - Anti-theft measures).• Do the sMMI contacts on the dock spring back properly? Pressthe 8 pins into the dock one after the other using your finger.Check if the pins spring back. If the pins are stuck, use contactspray.
I believe these bikes use CANbus comms , if you can't get it working then a dealer visit will be needed and possible firmware upgrade ?