This could be the cause of me moving files around. See I live at two places so when I decided to mod my mine craft I needed to carry my whole setup (because a good few hours went into my modding and I did not want to do it again) to another computer. So I copied my .minecraft along with this folder that I use to Hold my Magic Launcher (not really sure why I just wanted it in a folder and doubt that's a problem... >.>). Everything is the same as it was at the other place I live and NEI works there. So I bring it here today and everything but NEI was working.
I have the same issue, where the GUI will not show, although with a different mod setup a few days ago I was fine. While researching the issue, I saw a poster suggest that Better Breeding caused it for him. I also have Better Breeding installed as well. Just sucks since in this case I hadn't backed up my .jar
(Luckily, I think BB, Forge, and Optifine were my only jar mods... Going to attempt a clean jar with just forge and optifine, and see how it goes)
Edit: Took the lazy way out, copied a jar from another MultiMC instance, one without Better Breeding. Got the splash about the missing better breeding stuff, and whether I wanted to continue. Clicked 'yes', everything loaded up fine, and NEI is now working as intended. There definitely seems to be some compatibility issue between Better Breeding and NEI. (Just noticed this is mentioned in BB's opening post as well)
Again, your issue could be altogether different, but this shows the lack of GUI could be caused by a possible incompatibility (even with a mod that has no effect on GUI whatsoever), so hopefully the info may help.
The Meaning of Life, the Universe, and Everything.
I had similar problems with NEI gui. It happened after playing around with damage indicator gui options. First the GUI came up darker than usual, then stopped coming up altogether. First, I tried reinstalling damage indicators with no love. Then reinstalled NEI with no love.
It turns out that all I had to do was press a button under the main inventory area just above where the search bar usually is. For me, the button was part of campers mod. Unfortunately, I can't get you a screenshot because I can't reproduce the problem after fixing it. Listed all my troubleshooting steps because I'm not sure if it was one thing that fixed it or a combination.
Hey man, i was having this same issue before, but i figured out how to fix it. First go to %appdata%, then Roaming and go to the minecraft folder where NEI is installed. After it, go to config and open the NEI notepad archive. Then you are going to see something like "hidden=true", over there, just change it to "hidden=false", and then save it. Well, it worked for me, but i'm not sure if it's going to work for you. Good luck man !!!!
Ps: If you got the game open, you will have to close it and open again !!!!