Answer:   There might be plenty of reasons for the data loss in Salesforce. Some of them include
Migration of currency, number and percent from other data types changing to the multi-select list from other types except for picklists.
Altering to auto-number except for text.
Altering the information and date-time.
Altering from the checkbox, auto number, and multi-select picklist to other types.
Altering to phone, text, URL, and email from the text area.