If you did your 2015 tax return with the desktop version of the program, then the only place your .tax2015 file exists is on the computer you used to complete and file your 2015 tax return. If you did not make a backup copy and that computer's hard drive is no longer available or accessible, then it's gone forever. Hopefully you have a printout.
If not, then you can get a free transcript at https://www.irs.gov/individuals/get-transcript
If you want a copy of the actual tax return itself, then the IRS will mail one to you for $50. They will only mail it to the address on the tax return. (Hopefully, you haven't moved since filing that 2015 tax return.)