After changing choice column type from multi choice to choice back I have lost all values

I am not sure that changing the column back to a single-choice column would automatically make all values blank. Could you please share a screenshot showing how the column and data look now?

Based on my experience, when a Choice column is changed from single-select to multi-select, or from multi-select back to single-select, the column values do not typically disappear completely. In some cases, the values may still exist in the underlying data, but the relationship between the stored values and the available choices can break, causing the values not to be displayed correctly in the SharePoint UI.

Example:

It would be helpful to see a screenshot of the column settings and a few sample list items so I can better understand what happened and determine whether the data is actually missing or simply not being displayed as expected.