If you used desktop software downloaded/installed from a CD to your own computer, then the tax files are stored locally on your own hard drive or any backup device you used like a flash drive. They are not stored online. If you cannot recover them due to a hard drive failure, etc. you can get a free transcript from the IRS or for a fee of $50, an actual copy of the return.
https://www.irs.gov/individuals/get-transcript
Contact your state for copies of previous state returns.
https://ttlc.intuit.com/questions/1901670-how-do-i-contact-my-state-department-of-revenue
You would need the files ending in .tax 2015, .tax2016, .tax2017 and each years program installed to open them. If you bought the program downloads you can download them again. Or install from the CDs.
Or you should have also saved each year as a pdf file when you were done. Look on your backups for all files ending in .tax or pdf. They should have been in your Documents in a Turbo Tax folder.