This means your drive has either stopped responding or RST thinks it has.
I remember this happening to me once. Panic at the disco! I shutdown and unplugged the machine for 5 minutes (praying the whole time) and then rebooted and... everything was ok (whew!). There was nothing actually wrong with the drive and it has worked perfectly fine ever since. I think that, at least in my case, it is either a glitch somewhere in the RST infrastructure or I had a power issue (ground loop or something similar). Regardless, lesson learned; regular backups are the ticket!
After power cycling, go into BIOS Setup and see if the physical drive is (now) responding and is being properly enumerated by the BIOS. If it isn't, then you know you are seeing a real drive failure (sorry if that's the case).
Hope this helps,