Look in your main fs9 aircraft folder and if you don't see the default Cessna 172 there then you have either inadvertently deleted it or moved it to where the sim can't find it.
The thing is, what aircraft are left in the folder if any? Also what is the flight that comes up when you start your sim? Is it the FS9 default flight with the Cessna at Seattle or some other flight which you have saved yourself?
The trick now is to force the sim to use different a/c at start up.....not too difficult with a little thought and CARE.
Here's how it can be done depending on which aircraft you have left and which flight is the default......
First open the aircraft.cfg of the plane you intend to use and note the TITLE...exactly as it is after the Title=.
If it is the FS9 default flight you are using then go to the main FS9 folder and open the Flights folder...open the one marked Other and then open the FLTSIM.FLT file in word pad(This is the original FS9 default flight)......and find the Plane section.... change the
Aircraft=Cessna Skyhawk 172SP to the plane of your choice for example...
Aircraft=Beech Baron 58...close and save the change. The Seattle flight will then use the new plane instead of looking for the C172.
If your default is a different one then do the aircraft change in that flight instead...............
Make back ups of all files before any changes are attempted

IMPORTANT !...Don't try this unless you are sure you can handle it........commoner

BTW It might be easier to reinstall the cessna..but not half as interesting ;)