Hi Manpreet,
Data loss may cause due to following reasons
- Changing data and date-time
- Altering to percent, number and currency from other data types
- Changing from multi-select picklist, checkbox, auto number to other types
- Altering to multi-select picklist from any type except picklist
- Changing to auto-number except from text
- Changing from text-area to e-mail, phone, URL and text