I agree Revision Control isn’t the issue, but its super important to use it, unless your super lucky.
Each check-in becomes a snapshot, which you can open your map up to how it was at that snapshot.
This helps identify problems, restore deleted content, have a place to roll back to if things go cactus and unforeseen local hardware issues.
Make a snapshot at least when you finish for the day.
Go into UEFN Editor Preferences and in the Output Log section, enable the Timestamp mode.
Try to play, then look at the output log in UEFN hopefully it will show something. It may show a bunch of timeout errors… hopefully it mentions a .UASSET
If it turns up empty, then its time for a systematic troubleshoot… what devices, custom objects, AI, sequences, verse etc are in your project ?