If all she did was to delete the default flight you can remake it easy enough. If the opening menu comes up just go to create a flight change the location to the airport and gate you want select the airplane and weather you want and save make sure you check the box that says make default flight. 8)
If on the other hand she somehow deleted FS from your system you may have to reinstall it.
If she used control alt del then ended task it should not have removed the default flight. I do not see how she could have deleted FS in the way you describe. I would think you would have a error message appear the next time you start your system up.
If you use windows XP or windows ME you can also use the system restore function to go back before that day and that should solve the problem.
If you are not sure how to do this just click start programs; acessories; system tools; system restore; then select return to an earlier date .