If you have Start8, it will be unable to work properly on Windows 10. The path detailed below is how to obtain Start10 for Windows 10.
Start10 is now available to everyone for purchase, try, or upgrade. (It is not a free upgrade however)
If you have Start8 you can find upgrade pricing on the website below by clicking the "Own Start8?" link near the bottom of the page:
http://www.stardock.com/products/start10/download.asp
If you still have Windows 8, and you only needed to reinstall Start8, you can obtain it from here: