The only program that can reliably work with .key presentations is of course (iWorks) Keynote, the presentation app for Macs that was used to create the .key files in first place. Keynote has support for PowerPoint and can export .key files to PowerPoint's .ppt presentations, which essentially represents key to ppt conversion.
If you are on Windows and do not have access to a Mac with Keynote, it will be very hard for your to get your keynote in PowerPoint format. However, some latest versions of LibreOffice should be able to open Keynote presentations in the Impress program. If you at least have Apple ID, you can try the online version of iWorks.