Did you use the HOLES tool on accident by chance? If so just click on the HOLES tool again and hold SHIFT + Left click on the missing area and it should reapply your ground for you.
Typically if you didnt save every ADT it would be a complete and perfect SQUARE. from the looks of the picture its not just a square. it looks more like a few of your chunks got turned into holes. so like i said above. just use the "holes" tool and turn them back. If that doesnt work then back up your adt's (put them in a different directory after you copy them) and try using the Taliis and ADTadder and just save the adt's again. don't regenerate them. just SAVE them. it will load the missing files again.
If still no fix after all of that send me your adt and wdt files and ill see what i can do.