External hard drive not recignised by pc

Hello,
recently my external hard drive decided to hide on my computer and every other pc i tried it on,it will not Auto play nor does it show up in "My Computer",however it is there in Device Manager ,i can't open it from there..
When it first done this it was showing up in "My Computer" but it said do i want to format,i said no as i would have lost all my data....please help...