Download the save file and follow the instructions to get a new save game with everything in the base campaign completed. If your data is already gone, there is another option that doesn't involve starting completely over.
If you don't, your Steam cloud save will be overwritten with a fresh save, sending you back to the very beginning of the main campaign. Now you're locked and loaded with your save data so you can get it back once a patch arrives. You must be logged into Steam on your web browser not on the Steam app for the link to work. This one is more of a workaround than an actual fix, but everyone should do this to avoid any potential problems. If you're having the Iceborne won't load on PC bug, check out our current list of fixes over here. Before we look at the Monster Hunter: World corrupted save gave fixes for Icebornekeep in mind you might not be able to load the game at all after installing the latest update! This is a persistent issue on PC, but there are a few fixes and workarounds to try so you don't have to start over from scratch.