My Sketchup 2023 file won't open and has an error message

It stopped backing up automatically for some reason. I don’t recall disabling it.

Look in the same folder as the .skp you uploaded. See if there is an .skb file in there with the same file name. It would be in the same state as the previous save you did, not the one from a week ago.

Saving to an external hard drive may well have caused both the corruption and the backup failure.