When playing Co-op in Mechwarrior 5: Mercenaries, if you get a mission that starts from a dropship (rather than just spawning directly onto the map), you sometimes lose mouse control when it’s time to exit the dropship. You can still fire, and the mouse will work on the game menu, but you can no longer aim using the mouse.
What worked for me
As an Nvidia GPU user, pull up the Ansel overlay using ALT+F2, then close it again. Mouse control returns.