fs crashing

hi, when i play FS, at some points, it will unexpectedly sound that windows generic 'ding' error noise, and then the only toolbar dissappears except for the PMDG and Modules tab. I am unable to exit flight simulator, as if i push quit, ctrl alt delete, or any other things like alt+f4, i just hear the error 'ding' and the game stays as it is. This is frustrating and i want to get to the bottom of it. Help! :(Alex