Missing HDD

[Monkeyman]

I have 4 HDDs and 1 SSD in my case.  The SSD is OS only and the 4 HDDs contain my vast collection of various TV shows (mostly complete series) and movies.  All 4 HDDs were shown (and working) on my desktop earlier today.  Now, I'm missing one.  I tried rebooting but that didn't add the HDD.

What can I do??  I need this HDD.  As money allows, I plan to add extra HDDs to back up what I've got but 1TB HDDs aren't cheap.

Thanks for any help you can give me.