Hey there, Krishna!
I suggest you try to run CMD - Command prompt (as administrator)
Type chkdsk x: /r (Where x - stands for the drive letter assigned to your external HDD)
This will scan and fix your HDD of any bad sectors.
You can also try the external drive on another computer and see if it makes any difference and gets recognized.
If this doesn't help, the sudden RAW format indicates that the disk has failed or has corrupt files on it, so the OS cannot access it. Unfortunately, there is no way to convert the file systems (RAW to NTFS) without formatting the HDD (this whole process is a formatting itself).
My suggestion would be from now on to always backup your data on at least 2 different locations (backup storage/cloud storage, etc.). This way you will be able to prevent such unpleasant loss of data.
Hope I helped!
SuperSoph_WD