Capcom's Monster Hunter: World is still its best selling games ever, shifting more than four million copies since its release in January 2018 and its big expansion, Iceborne is a big seller too. But it's not without its problems. Some players have faced performance issues related to optimization, and a nasty save deletion bug which has caught a number of players out. Fortunately, Capcom is addressing the issue with alacrity and has promised that an impending patch will clear the problem right up.
Partly due to player investigation and partly based on Capcom's own assessment of the problem, it reportedly stems from a save file update that Capcom completed in October 2018. It changed the file format of save files and that means those who didn't complete this update because they didn't play the game at that time or since, have now faced problems where Iceborne can't detect their save.
Iceborne didn't break the save, it just doesn't know it's there.
Fortunately, that's a pretty simple issue to fix and Capcom has it coming down the pipe. It will bring Iceborne to version Ver.10.12.01. and should convert all save files to be the correct format. It will also fix the high CPU utilization error which has plagued a number of Iceborne players since its launch.
If you do try to play Iceborne before then and face the save issue, do not create a new save as you are prompted. Shut the game down and wait for this patch.
It should be coming in the next few days.