Why do I have to start from scratch, even though I've saved?

I save my progress, and when I return to the game and load the chapter the map I filled in has reset to blank!? Am I missing a load game screen/room somewhere. I cannot play any further until I know I can load my progress.

I don't know if this is caused by my Wii U freezing now and again, because other games' save files have been ok even when the system hangs.

It is likely to happen due to the freezing. You should figure out why your Wii is freezing.

Knytt Underground doesn't save immediately when you activate a new checkpoint, so if your Wii U freezes (which it shouldn't), there's a chance it won't have saved your game.
If you want to make sure the game is saved before your Wii U freezes, exit the game or power down your Wii U. That will ensure that the game is saved.

Also, send your Wii U to repair. They're not meant to freeze.