I have a problem with CRJ 700 and it is that with a route already established and in the middle of the flight the NAV button disconnects itself and does not let me activate it again
Del and reinstal
Check if the AP disengaged. If that’s the case, you have to put the AP disengage switch back to the neutral position.
Did you touch your flight controls? Sometimes I am pushing my yoke, when I am leaning forward on my chair. The airplane will think you want to override the AP inputs, so it will disengage.
The EFB has the option “Autopilot Disconnect Yoke Sensitivity” . I use the medium option for my Joystick.
Or do you push some keyboard buttons?
This would speaks for me for an addional keybinding that switch off Nav.
In that case search the Keyboard options in MSFS.
I modified your title to give a little more detail.