I downloaded a scenery file. Now I can only get the start-up scenery, no matter what I choose. What I mean is, if I choose a saved flight, it's ok, but if I want to change to another airfield, it won't change. I tried deleting the scenery I downloaded, but it did something bad.

Have you tried to delete that scenery item from the list under Settings>Scenery Library?

When you add scenery to fs it make a entry in the scenery cfg file if you delete the scenery from fs by using the add scenery menu in fs this will not delete the files in the scenery folder. Most scenery files will have two subfolders scenery & texture. Unless you had to run a program to install the scenery. To remove the scenery just delete the main folder where the scenery is at then make sure to remove the entry in the scenery cfg file and that should be that.
If you had to run a program to install the scenery eith then unistall it like you would any other program you want to remove. This is the only way to clean up this kind of scenery simply deleting the folders wont work.

Thanks guys, any input is welcome. I hate to do it as one never knows what gets lost in the process, but at a last resort, I'll do the uninstall, install thing. Basically, I never use any auto install programs, just unzip to a download folder and then make a new folder, drag and drop the scenery and texture folders to it. Then add scenery from the settings. Again thanks a million for all the input.