Unable to find model

This error means that model index is corrupted an issue that was not addressed in v8i era and still comes up. File is easily fixable at least you will be able to open file even if something else will be missing.

If you are not willing to share the file here is the instruction how to fix - https://communities.bentley.com/products/administration/f/product-administration-forum/144134/unable-to-open-design-file/447193#447193

Key-in: MODEL REINDEX would work but as file can't be opened this key-in doesn't work.

MaryB said:

They may have ways of restoring the file.

They don't they will say use backup, oh you do not have backups then you need Projectwise.

Dawn Michel said:

When trying to reference the file in a different file, we get "Unable to find model "(null)" in Control.dgn"

Does this happen after you select the file in reference manager and then try to attach or as soon as you try to select the file in Windows Explorer dialog? Does it at least load it into the reference attachment dialog? If you are able to select the file & it loads into the reference attachment dialog there is a model drop-down. Are you able to switch that and load the file with a different model selected as a reference?

You can open a Service Ticket with Bentley support. They may have ways of restoring the file.

Other than that, I can't think of any way to restore a drawing that is so corrupt that it can't even be referenced into another file.