Suddenly,Tivo files won't play on Windows Media Player

On Windows 10?
If so go into the Microsoft Store and search for "MPEG-2 Video Extension" then install it.

 

You haven't provided any details on the nature of the downloaded file, such as whether it retains a .tivo extension or was converted to .mpg (or other), or even the tool used for the download.

If it's a .tivo file, some update likely whapped WMP's head, and there's probably some trick needed to remind WMP of the codec needed to unwrap a .tivo file.

edit: e.g.:

 

Define Won't play? Is there sound but no picture? Then its MPEG4 and you need to download again with "Faster Transfer" setting enabled. In KMTTG, its TS download.

 

Have you tried the quoted suggestion?