if you edited a stock unit and it then became invisible (with other units being visible), it may be because your builtmods\cnc3xml folder contains the wrong data and as such, the SDK cannot find the art it is looking for.
Delete everything in builtmods\cnc3xml and replace it with the 2 files in
http://users.tpg.com.au/jfwfreo/CnC3Xml.rar and only those 2 files. Also, make sure you NEVER delete builtmods\cnc3xml or these 2 files otherwise things WILL break.
also, if you are affected by this, delete the entire "builtmods\mymod" folder (or whatever your mod is named) and rebuild the mod.