I have never encountered this sort of problem, but I have heard of it and would like to know how to resolve the situation in case i do have to deal with it...here it goes: What if one of the channels of a file got lost or deleted..how would one go about replacing a color channel? (cyan, magenta etc) Anyone??

If a channel has been deleted the image with revert to MultiChannel mode. You can right click on one of the two remaining layers and select Duplicate Channel. Then do Image>Mode>RGB . You will have an RGB image but you will have replaced the missing channel with one of the remaining two. The color will be wrong. If your image had been closed or the history buffer over run, you can not get back the original channel and there is no way to recreate it unless you have a copy of the original image.
Regards, Murray