You should never format a disk that contains data you want to keep.
Format is a write operation. It writes an empty filesystem to the disk. When you format a disk in the array, Unraid treats this write operation just as it does any other, by updating parity. So, after formatting a disk in the array, the only thing it can rebuild is an empty filesystem.
Not enough information at this late date to know exactly what happened.
If you had asked for help when you were having problems maybe we could have saved your data.