there is no module package to reinstall it -the easiest way to fix this is to create a package for it i.e. install another version of dotnetnuke (matching the version you have), go to host->extensions, edit "authentication", scroll down and click "create package" and use that to install on your borked build (you might want to check the desktopmodules table for "authentication" and the modulecontrols table for "Account Login" first though i.e. if "authentication" is missing create a new module definition and add "account login" as a module control

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Thanks again. Created the package without a problem I think.
Now confused about what to do with DB.
Before attempting to install package on broken version, i looked at DB as suggested.
- Desktopmodules table does not have an 'authentication' record.
- ModuleControls table has "account Login" record but it has no moduleDefID.

Should i now create a new ModuleDefinition record, call it "authentication" and enter its ID in the MOdulecontrols table as the moduleDefID for the account Login record? Then proceed to attempt to install the package?

Thanks.
Hans