Playback for videos gone black when adding a new video to timeline

That's a great question, being new to this I wasn't even aware where the backups were until I talked to Apple, which, were a lot less helpful than this forum so thank you all! I had already reset preferences & I have tried multiple backup files (by selecting the 4-19 date under a block of Untitled_202004##_####_MST file. Should I be selecting the Untitled part not the star icon w/ the 4019 below it? Which is the backup?

iMac (Retina 5K, 27-inch, 2019) 3.6Ghz 8 core i-9 w/ 16gb 2667 DDR4 Radeon Pro vega 48 8GB. Thoughts? Needs more ram? Sure.. but it should be more than sufficient to crank out some video if I wait on it... I do see the spinning wheel in the top rendering... not sure.

BTW rules for this project import proxies and optimize the clips based on the 422 LT for editing.

I do see also that when I switch between "optimized/originals" and the proxy that only SOME of the videos have proxies. When/how does it create proxies if not on import? I double checked a couple videos too and I see the similar marking in the info inspector that only original is green and the optimized/proxy are not available?...

I've gone back to a backup and it keeps happening- doesn't matter what video I try to add. Sometimes it will allow me to add a new one and then does the same thing a few videos later. I don't believe I hit V.....but how would I reverse that if by some miracle I did? The audio plays back fine

sorry not solved - I did check the files to verify the rendered proxies and I went ahead and kicked off the transcode proxies and optimized again to get greens on all the files. I finished that now but still have black screen in the preview window. Now that I've done that I have an exclamation mark in the 4-19-20 star folder.... what causes that?

I may have determined what the issues was... even w/ our brand new iMac with awesome video card, it seems that the way abobe premiere was causing the CPU to overheat and shutdown constantly, we're running into a processing capacity issue to parse the video clips and render over 100 (even optimized and proxied) clips on the same preview pane at once. We can let it sit and render but it still won't eventually display... That said we continued adding the remainder of the video clips and did an initial resizing and cropping by moving that video outside the track of all the other 100+ video clips on the timeline. That allowed us to see the video in the preview pane. Then moved it back, and screen blacked. I'm pretty convinced this might be the issue also because then I tried the compact clip option of say 20 clips, and immediately all the video clips appeared in the preview pane. We tried also deleting videos off the timeline down to 99 and the preview pane displayed fine even dubbing and scrolling through the timeline with video and audio seamlessly.

The issue seems to be that, either with our processor core usage, or the video card usage 8GB RAM along with our 16GB RAM that the processing at that point became too much. Has anyone else experience this? This project does seem to be a bit more large scale video editing than any normal person might do... I think our path forward is to sync the audio and then compact clip multiple videos to position. We want to show all 130 clips at once, then fade to individual clips of some 20-40 clips at a time, then end again with all 130.

Any additional tips are appreciated. We read about audio sync, we can try to do that as we had everyone start with an initial BEEP in the video to sync. Not sure how that works yet.

osCatalina- iMac is less than a week old with a Pro Vega Video card. FCP is 10.4.8 Final Cut Pro X

The audio plays just fine and the videos will play individually but not in the area where I have cropped and resized them all. Sorry for not knowing all the terminology, I'm 2 days into this program.