What version of AE? Did it ever work? Did you try resetting your external monitor to Mirror? Do you see the AE program icon bouncing in the dock when you try and open AE? Can you see the AE icon when you use Ctrl/Cmnd + tab to switch between programs.
The only time I had this happen with my MacBook pro I was editing with a 4K monitor connected and I'd moved AE completely to the 4K monitor, then I forgot to move it back when I shut down and went back to my studio. I set the display preferences to maximum resolution and then reset them and nuked my AE preferences on launch and it showed back up. I'm on Sierra now and have not had the same problem again.
I tried the 3 latest versions 14.0.1,14.0.0,13.8.0
Also tried mirroring the monitors, still nothing.
I can see the AE icon bouncing when it opens and I can cmd+tab to switch to the application, but none of the windows show up at all. It's very strange.
I don't think El Capitan has that assign to function, it doesn't show up for me.
Apologies for this frustration. Are you still experiencing this UI weirdness? I suggest you work with our agents on this issue. Please let us know the solution if you found one.