Access Shows Unrecognized Database Format: What Not to Do to the Production File

When Access shows Unrecognized Database Format, do not begin repair attempts on the only copy. The error can come from an incompatible version, a misleading extension, a partial copy, encryption, or file damage. First stop users and make a read-only copy of the file and relevant log folder.

Record the file name, size, modification time, Office version, network path, and the last action before the incident. Open only a copy, test whether a same-day backup opens, and see whether the issue occurs on another workstation. This separates file damage from a permissions, disk, or software-version problem.

Even if the file opens again, do not simply continue. Reconcile tables and record counts against the last report, then build a backup routine with restore testing. A database that has never been restored successfully is not an operational backup.

Source

Microsoft Support: Access