If the HDD fails and the data is lost, obviously next time you'll have to re-download everything.
A word of advice, if your computer also has a SSD there are some tricks to make the whole process much much faster (at least temporarily keep the chainstate folder on SSD, maybe the indexes too).

Windows reinstallation is a different discussion. You can locate the data folders(or maybe you configured it from start to not be on c:), back them up before the OS reinstall, and next time use that folder as datadir. Then it will just continue/sync from the point you left it at the previous stop.