How do I solve iMac "Bluetooth: Unavailable" problem?

I have a 27-inch, Mid 2011 iMac (OS X Lion 10.7.5) which was provided with a wireless keyboard and mouse. Afer a year of use, I started having intermittent problems with the keyboard becoming disconnected, but the problem usually solved itself after a few seconds. Now, I am not able to use the wireless keyboard and mouse at all. I installed new batteries in both the keyboard and mouse, however, it did not solve the problem. The Bluetooth icon at the top right of the monitor reads "Bluetooth: Unavailable" in light gray lettering. I am currently typing this with a wired keyboard and wired mouse. I cannot find any solutions in the Apple Support section. How can this problem be solved?