Why would all 4 of my shrimp just disappear/die?

[tabitha123]

After having one for several months, and he was doing good. I decided to get a few more.
I did a water change a couple days ago and havent saw them since. I've took a flashlight and looked all around my rocks, none of them to be found. Could something possibly wipe them all out?

 

[James M]

How big is your tank ?
What inhabitants do you have ?