Q: usb Ext HD  not visible on desktop

hi  i have a Bipra 500GB  Ext HD    which is connected by USB 2 port. i use his as my time machine backup disk

It was visible and working till last night

now i cannot see it  on the desk top  although it's visible if i looki in disk utility   and it is powered up  ok    .

I can't see why it's invisible ,

Advice would be appreciated

Thanks