I accidentally closed disk defragmemter when it has just started defragmenting external WD HDD of 500GB, now whenever I connect the HDD to my laptop it does not appear in my computer explorer but at the same time it is recognized by WD shutdown or stop utility.

I am therefore not able to access my data on the HDD.

Can anyone help? I have around 350GB of data on it.

Seems that the ext HDD needs to be repartition again to see the drive letter in "My Computer".

before doing this, please backup the data to the other Pc, may try slave the ext drive on the other PC and use 3rd party software like "easy recovery" to back up the data.
Please be caution when using these 3rd party data software, here I just got you some suggestions.

Hope this helps.
Bill
tech Manager

Your WD HDD of 500GB drive is corrupted due to accidental close of disk defragmenter so now you need a data recovery software for the recovery of your data and files, i will suggest you to use Kernel for FAT and NTFS hard disk data recovery software for the recovery of your data. It will preview your data in scan mode and will ask you to save all or some selected data to your location. for more details regarding the recovery of data go through - Windows Data Recovery - NTFS data recovery - Partition Recovery - Recover Deleted files