Sounds like you've corrupted the file which controls the objects such as airfields but it's hard to say. It could also be the country file so try that first, it will save deleting all your field work if the TE doesn't crash. Remember to save often and re-open your terrain, or restart the TE as is often recommended.
Find your project folder:
C:\Program Files\HTC\ah2editor\ahiiterr\
Delete the file .FLD, this is the binary country file.
See if you can run the TE, if that works, be sure to reset the field ownership like MachNix said; Menu-Settings:Fields.
If that still doesn't work... Rename the folder to anything else and relaunch the TE. This will keep the TE from loading your corrupt object file. Create a new project with the same name as your old project, exit and copy your elevation and type files and texsrc folder into the new project folder. You should have your elevation and tile-types intact but you'll loose all your fields etc., probably just as well anyway. Save the terrain and start placing your objects again. Hope this makes sense.
