What happened?
The filament has been ejected to the front of the MMU unit so that you can remove it.
This feature is handy in case you are using the Spooljoin function. A depleted filament might be too short in order to be removed easily from the back of the unit. By ejecting it to the front of the unit, you can easily grab it and discard it.
How to proceed?
Remove and discard the ejected piece of filament and hit the button on the LCD screen.
If you accidentally triggered the eject function, push the ejected filament back into the MMU unit and make sure it isn’t protruding out of the short front PTFE tubes - so that the selector doesn’t jam later on.