Are the maps that are gone still inside the osu! folder, i.e. find a < 4.15 star difficulty or map, download it, import and see what it says.
Usually it is the star difficulty display setting but you've not given us much to work with.
What did you do prior to this, if you have accidently deleted maps below the threshold based on something else and they arent appearing in your recycle/trash bin then they mightve been lost.
Did you get any errors or anything regarding this?