In our PROD model, mistakenly ALM Structure was disturbed. Is there any option to revert back the ALM Process Structure and we cant restore the model because the user has inputted a huge volume of changes.
As a Workspace administrator you can only restore a model back to a restore point ID that existed after the model was last synchronized and revert all the changes into Dev and synchronize it back to Production.
There is a reason why Anaplan wants you to confirm if you want to take model out of deployed mode - it is risky. As explained by @Jitendra.Vishnolia you can do it either way.
If you don't want to restore the history and then you can go to Prod Model - Revision Tags => Revert to Last Revision. It will undue all the structural changes in the Standard Prod Model, post which you can put the model in deployed mode and that should solve your issue.
But If there was a RT created directly in Standard Prod model then you will have to make a new Dev model from the "Create model from Revision" Option under Revision Tags and use that for ALM.
in short no, you can't roll back the structure and not the data. Often, data is harder to re-input. Un-deploy your model, make the changes manually, create a revision tag, deploy the model, create a new model from it (or make a copy) and make that new model your new DEV and you are back on track
Have you created any new revision tags in your dev model since syncing the previous revision tag? If not it is likely easier to create a new dev model using your existing prod and put ALM back in place using these models.