Make sure your all your xml files for that particular cell are correct and both converted well. You need both (your).bmp and (your)_veg.bmp in one folder, locate it and drag drop, than convert it with the option (assigned generated maps to the world) and once you do that properly and no file is missing there shouldn't be a reason in the world that you double click and get grey.
Maybe you moved your files and than re-opened project so it lost direction.
Or maybe your specific bmp/_veg.bmp have some kind of error due to coloring mistakes. Check in all that, other than that there shouldn't be a problem in the world.
Also check that for each cell the the bmp is exactly 300 pixels per cell.
If nothing works, either make a new project from scratch and start converting veg files until you find the corrupted one, once you do make sure to input proper colors, and if that doesn't help, check if your WordEd has something corrupt within it. The way you do that is by making a pure white _veg, it should give you errors but still run, if it doesn't read that than your WorldEd is corrupt, in that case reinstall it.
Make sure you have an updated version of the tiles, to what ever build you're using. Because it might just not locate the grass dirt and your "floor_" tiles in general. To do that you go to Tilezed pick "tools" choose "Tilesets..." and check that all your canvas/floor types aren't red.
Encrypted's post in Nothing appears in cell was marked as the answer