I just ran into this today. In my company title block, the company name is changed to the Calibri font. This font suddenly became invisible. I could change it back to the native pads font, but when I went back to calibri it disappeared. The cause of this was due to a dxf block I imported into the database. When I exported the dxf file, I must not have selected the correct format and when it imported, the Calibri font was not available so even though this text had nothing to do with the import and already existed in the job, Pads made it invisible to satisfy the dxf file.
Ascii out the job, search for Visibility and near the bottom of the file you will find a section titled Visibility Data. Below that (in this case) is a line "Font_face Calibri". I deleted the word Calibri, saved the file and when I imported it into Pads, the text reappeared.
I don't know the scope of this visibility data section of the ascii file, but I suspect it can be used to control other things as well. There is nothing I could find in the help files about setting the visibility of text fonts, so I don't think this can be changed through a layout tab anywhere.