Why was the level in the Fortnite Lobby above my character reset to Level 1 but my progress and items are still on my account?

If the level you see in the Lobby screen above your character is reset to level 1, and you still have all your items in the Locker, a new Season in the Fortnite Experience you're playing has started. 
You'll need to play the new season and level up again.

Remember that your overall account level will not be reset, and you can see that level in the Careers tab in Fortnite.

However, if your account was reset to Level 1 and all your items and progress are missing, please see I was reset to Level 1 and my progress is gone. Was my account deleted?