LucyLovesMarvel There are going to be at least a few notes in the Reliability Monitor. Do you mean that you didn't see any Sims-related errors, or that you didn't see anything at all? If you can't see any info, you're not looking in the right place.
For the old save, how large is it? You can see the size in Documents > Electronic Arts > The Sims 4 > saves. And do you use Windows Defender or a third-party antivirus? Defender's Controlled Folder Access sometimes causes problems with the game, and another user recently said it was blocking them from saving their progress in their large save due to the sheer size. But a new save would be much smaller and presumably be unaffected.