This is a bug, but I think there is a fix
I've run into this bug myself. Some features of the text are just hard to change for some reason: for example, I have difficulty changing the color of the text through the "Drawing Tools" (you can change it through the format menu much more easily).
But when it comes to the text being invisible, I think I've found a fix. That problem seems to go away if you close all other Mathematica windows (meaning any other "notebooks" you have open). Once you've done that, you should be able to change the opacity of the text with the slider, and make it visible again. Keep in mind, changing the opacity will only make all your subsequent text visible. Your previous text will still be transparent, and should probably be deleted.