You probably were using the drive to save more data or write data.
When you delete a file from a HDD the original data is not deleted but rather "delinked" from the system (to talk in simple terms). This way the PC thinks that the sector that the file was in is empty and used it to writte new data on top of the old data. It overwrittes the old data you "deleted".
Therefore, after a year, taking into consideration you are using the drive, the writte of data in the drive when the PC shuts dows and starts up and many more variables it is impossible to be able to get your files back. UNLESS you haven't touched the drive from then.