Downloaded file doesn't play

Nothing you did wrong - corrupt files are an indication of a bug in MuseScore. It looks like this file was created in MuseScore 1.3, so it's probbly a bug that was fixed long ago. MuseScore 1.3 would unfortunately create corrupt scores and then not detect that, letting the problem remain and indeed often get worse and worse each time you edited the file. So MsueScore 2 adds a check for corruption to give you a chance to fix the problem right away, before it gets worse.

Anyhow, to fix corrupt scores, see https://musescore.org/en/node/54721. There are a number of tricks described there, but in the end, often the best solution is to simply delete the offending measures completely (ctrl+delete), insert new ones, and re-enter the notes.