Unable to restore iphone from computer

My iphone 6 developed an error when I was backing it up to my computer with a very full hard drive.

I dont think it was able to back up properly.

Then the iphone died!

I have rectified the overfull hard drive on the computer but am unable to restore my iphone.

Error Message = "itunes could not restore the iphone because an error occurred while reading from or writing to the iphone"

Thanks in advance for any assistance.

I have never backed up to icloud!!! I use dropbox to store photos etc