7-port usb 3 hub stopped working osx yosemite

Have been using this unit for three weeks on a Macbook Pro Retina (2013) running Yosemite (10.10.3). I run up to four USB 3 devices thru it, including 3 external hard drives. Never had a problem but today, after the computer had been to sleep for several hours, none of the hard drives were recognized by the computer, though all the blue lights were lit on the hub.

I disconnected and reconnected everything several times but hard drives never were recognized, even though their “working” lights would come on and you can hear them reading the discs.

Restarted the computer but no difference. If I plug the hard drives one at a time directly into the Macbook they are read and run normally. If I plug into the USB 3 hub nothing is recognized, but the blue LED lights on the hub come on.

Any ideas why this unit would suddenly stop functioning?