How can I reset my Di2 system?
Unfortunately there is no such thing as a Di2 reset - the closest you can get to a full reset is to disconnect the battery for 30 seconds and then reconnect it. That will cut power to all components and restore them to a somewhat 'neutral' state.
Having done that it can also be a good idea to connect your bike to a Windows computer and then reset all values to their defaults.
For 12-speed Di2 bikes, be sure to disconnect the battery middle port. Even if there is just a dummy plug in there, just unplug that, wait 15 seconds or so, and plug it back in. That should bring your 12-speed bike back to life.
Note that you cannot easily connect 12-speed bikes to a Windows computer - the SM-PCE02 is required to do that.