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Dear Serenity,
From the description "my map has disappeared with a tiny thumbnail appearing when I view in 1% but there is nothing to see when I zoom in. The diagram is completely corrupted/gone.", it seems that the shapes are corrupted by a crash.
If it is convenient, I'd like you to confirm whether you enabled AutoRecovery in Visio. If yes, check whether you can find some possible versions of the file in %userprofile%\Local Settings\Application Data\Microsoft\Visio.
How to turn on AutoRecovery in Visio - Microsoft Support
From the information "I am saving in one drive as I go but to no avail", it seems that you also can't open the Visio file with Visio for the web. If yes, I'm afraid that it may be hard for you to recover the Visio file from your side.
If you are a Microsoft 365 business user, as another chance, I suggest you contact the administrator in your tenant and let him follow the steps in Get support - Microsoft 365 admin | Microsoft Learn to open a support ticket to our related team as soon as possible. If the file can be sent to them, I think that the next step may be to send the file so that they can check whether the issue can be fixed from their side.
About admin roles in the Microsoft 365 admin center - Microsoft 365 admin | Microsoft Learn
How do I find my Microsoft 365 admin? - Microsoft Support
The administrator should be able to open the support ticket from Help & support in the Microsoft 365 admin center.
Then you can select the Phone support or the Email support as follows.
Thanks for your effort and hope that the data can be recovered by our related team.
Sincerely
Cliff | Microsoft Community Moderator