Same thing happened to me too this afternoon.
Went to add a HOLD at the first waypoint to the star. FMS listed 2 waypoints of same name within 60 miles of each other, so wasn't sure which one to choose, so went back to the LEGS page to check on the distance to go to the waypoint. Then when I attempted to go back by clicking the HOLD button again, nothing happened. Then discovered that absolutely nothing worked any more in the aircraft. Complete system freeze.
The aircraft was still flying and descending, as I was at the time, but none of the displays were updating, none of the control panel buttons had any affect, couldn't disconnect the autopilot, engine gauges were frozen on the display too, basically I was just looking at a frozen picture of the whole cockpit.
This is the first time that I've flown the CRJ7 since MSFS2020 SimUpdate 6, so I'm thinking that something has broken post update. Unless I've just been lucky in the past.
Please please investigate and develop a fix as this type of complete lockup simply makes the plane unflyable.