I got to level 28 then when I joined my friends I got reset back down to level 1. Turtle Rock is lucky I enjoy this game so much or I would have traded it in already.
Or, ya know, you could contact 2k support so it could get fixed. Like every other person who has posted with this issue on the boards over the last few days.
There is a fix in the works and they are aware of the situation. There is a step you can do in the mean time that may be able to assist you.
I hope this helps you and I am sorry you had to go through this. Best of luck too you!
thanks for the help
Also here are links to the two main topics about this issue:
If there are any updates you are likely to find them there!