Why is my clip broken?

See here:

https://community.adobe.com/t5/premiere-pro-discussions/faq-what-do-i-do-when-i-see-diagonal-lines-on-clip-s-after-moving-media/td-p/10391911#10986508

Usually, this happens when PPro relinks your clips to other clips that doens't have enough media (shorter in duration).

To fix the problem, try this: in the project panel select all the problematic mp4 files: right click > Make Offline, then again: right click > Link Media, and relink one of them, PPro should find all other offline files and replace them correctly in the timeline (supposing that you still have those files on your HDD)