Logic Pro X MIDI Stopped Working!

Hi All! First off, my specs are as follows... * iMac Retina 5k 27" Late 2014 / Processor: 3.5 Ghz Intel Core i5 / Memory: 32GB 1600Mhz DDR3 / Graphics: AMD Radeon R9 M290X 2048 MB *

I have been using Logic Pro X (10.2.3) for about a week now. I've been composing/recording a few projects, using my KORG R3 as a midi controller for the software instruments (connected directly to my iMac via USB).

Up until now, I have nothing but success. I've recorded and mixed multiple tracks in 2 projects (originally created in garageband, but now saved as logic projects).

I have also added some audio tracks that I have recorded vocals and other instruments on, using a microphone that is plugged into my ZOOM H4N (my audio interface).

Tonight, I added a new "software instrument" track to try and experiment with adding some Cellos. No sound was coming out through my headphones when I record-enabled the track. When I play back what I recoded earlier, the audio comes through just fine, on all tracks. For some reason, I can't add or record anything new with my midi instruments, on new or old tracks.

There is midi activity in my transport window when I press the keys on my R3. Just no sound. When I start recoding on whatever track I choose, the scroll bar moves and is illuminated red, but no sound is heard and no new notes and regions are created. All of my settings are the same as they've been for the past week. The only thing I changed was I adjusted the tuning on my midi instruments to match the tuning of an acoustic guitar that I recorded with my microphone.

Another detail I can provide is that if I select a pre-recorded region in a pre-existing track (with notes already on it) WITHOUT record-enabling that track, when I press the keys on my R3, notes are added (without sound) to the piano roll, stacking on top of each other. I know that sounds strange, but it's hard to describe any better than that.

I've tried restarting my computer multiple times. I've turned my R3 on and off multiple times... yes, it is on. I've unplugged my R3 and plugged it back in multiple times. I've disconnected and reconnected my ZOOM (not that that should have anything to do with the midi functions). I've tried resetting the midi drivers in preferences. I've tried resetting the midi in/out in my transport window.

This seems to be haunted. Please help! Thanks in advance!