The Joker seems to have played a mean trick on Injustice 2 players. According to the forums and the Reddit Community, there's an insidious bug going around that affects your character's progress. Apparently all the hard work you've been putting into leveling up your character is lost when you leave the game and play it again at a later stage, leading to devastating feedback from the fans.

Netherrealm Studios is reportedly working on a problem, but Warner Bros. Game Support have issued a solution to work around the problem so long:

"...several customers on both PlayStation 4 and Xbox One platforms have called in regarding the issue, and the developers are aware and working towards a solution. In the meantime...{support} had fixed every one of the calls by having the customer perform a reset on their console to clear local cache."

Generally that just means powering down your console, waiting about half a minute and powering up again. If this doesn't work after a few tries, then contact WB Support so that they can help you retrieve your lost progress directly.

Source: Kotaku