Eagle Cad .brd error open file
Most likely your file was corrupted, either during save, or a crash. In your case it looks like you corrupted it when you renamed it.

Fortunately Eagle makes periodic backups of your files, and also a backup in the event of a crash. These will be files with the same name as your board file, but with an extension of .b## (crash backup) or .b#1/.b#2/etc. (auto-save backup).

To recover from one of the backup files, simply rename the file to .brd and open it. You should pick the one with the most recent date, hopefully it is quite recent. You will have lost any edits between when the backup was made and when you were working on the board, but if the backup is very recent, this shouldn't be too hard to rectify.