To even get the world to load, you have to delete <save>/HardcoreQuesting/players.dat (not the main Minecraft player.dat). Once you can load the world, you can get a fresh clean quest book, and use /hqm edit to get an edit-mode book that will let you shift-click quests to complete them, so you can bring your progress back to where it was. As
@jordsta95 points out, there might be some quests that don't unlock until you have enough reputation - but as long as you claim the rewards, you
should get that back as well. I haven't tested on this release, but had to do that from 0.5 to 0.6.