Hi there,
The Red and Blue server has recently merged to make up the Silver Server, and the Yellow Server is now renamed as the Gold Server. Therefore, you will need to play on the Silver server if you wish to continue your Blue server progress. You will need to merge game data files from both servers in order to play on the Silver server. You have an option of keeping both accounts if you have 200h playtime on each save file.
Please note that even if you only played on one server, you will have to do the merge to remove the unused account from the server. You can do that on the dashboard. For more information regarding the merge, please read the Server Merge FAQ. Please note that some accounts will not be able to merge if you have more than 1000 Pokemon combined, or if you were banned on one of the servers; in this case, you will need to keep them as separate accounts.
If you encounter some issues, be sure to check out the Common Merge Errors: FAQ before making another post.
Kind Regards, Astraea.