Loop Precomp

I'd guessing your comp might have an empty frame or the time-bases don't match.  LoopOut is good for what it does, but here's a video I made showing how to use it with a fade, so that your comps blend from one to another:

https://youtu.be/fXT8uUIkuP8

And what tutorial exactly? Posting a Google search string is not that useful as apparently it pops up multiple suggestions. that aside the typical issue with looping (presumably time-remapped) stuff is that people don't create that extra keyframe one frame before the end to "pin down" the timing and then things go black due to how AE samples the actual frame based on genuine time. So if that appears to be the issue, try this first. Anything more will require a correct link and a better explanation.

Mylenium