Naughty Dog Announced on their site that they have released a new PATCH 1.23.070 for Multiplayer.
The main reason is there was a bug When the Ranked Season 5 was launched on April 3 Players were facing an issue, Their progress being reset.
Here is Patch logs:
No rule changes this season but we are considering changing the ranked game mode starting next season.
We apologize for the inconvenience and look forward to seeing you online!
BALANCE AND FIXES
Added death region to Village to prevent exploit
Increase LP cost of Drake’s Aegis to 3 LP
So because of this they are going to launch a new Ranked Season today,
The progress of Ranked Season 5 Progress will be removed Sorry,
But do not worry Nuaghty Dog say Ranked rewards earned will be retained.
You you will able to download the latest patch.