I copied calc document and altered the copy but somehow original was also altered and have now lost a lot of data. Am I able to retrieve this data?
If you have a backup of course you can use that. In the unlucky event that you don’t have regular backups, if you haven’t yet closed the document since altering the copy, you might be able to use Undo - Ctrl+Z - to return to an earlier version of your spreadsheet.
Maybe you clicked File > Save a Copy, in which case the copy of your original file will have a new name or location and a new date (date of save a copy). Can you search for that on your computer?
Save a Copy: saves a copy but keeps the original open to work on
Save As: saves a copy of the original under a new name or location as the open file and closes the original