Preview movie keeps dropping!

I am a self taught newbie, trying to put my comics in motion. My first episode is 3,766 frames long, so I expect it to take some time to render. But first time i went to preview movie, it took 7 hours to render what turned out to be only about 4 minutes in real time… Then went back into the project and lengthened it by adding more and adjusting the velocity. Weeks later Im finished again and go to preview movie, and same thing…Hours to render, no prob…BUT somewhere down the line the export bar disappears and there is no preview. TBS is still open, not crashed.I have done this three times now with same results! Over ten hours wasted! Ive gone over every scene with a fine tooth comb and cannot seem to find a culprit. Everything is in a media file and renders fine when I preview each scene… But waiting 4 hours to get no results is killing me!
I am working off a MAC with toon boom version 6. If anyone has any suggestions, please shoot em my way.

Sounds like the project has become too heavy. Try breaking it down into smaller chunks to render and then put together in a post editing application.
You may consider upgrading to more robust software such as Animate/Pro if you wish to do complex/heavy scenes.

Make sure to make backups of whatever work you have at the moment so that when you are working on the rendering, you do not loose your hard work.