Your last year's return would be a .tax2023 file, which needs to be saved on your C drive. If you saved it to a cloud-type storage, when you download it to your computer, it may be a zipped file and/or corrupt.
Look for a backup copy on your computer. You may have several, if you only saved your completed return to cloud storage.
If you used TurboTax Desktop and still have the 2023 program on your computer, open it and save your return to your computer, then open it in your 2024 program.