"No Audio Output Device is Installed".
This means that your sound processing device has become disabled or uninstalled. Assuming you're (a) on windows and (b) you've checked the volume isn't turned down, no leads have become loose etc. then do the following;
Click Start. Right click my computer. From the dialog box that opens, click manage then on the window that opens, click device manager. Scroll down the list until you come to sound, video & game controllers. Expand that tab by clicking on the + symbol. If your device has become disabled or uninstalled, it should be listed there with either a yellow question mark or a red x by it.
Right click it & choose either update driver or enable this device. If you choose update driver then windows will ask you whether you want to install the software automatically or from a specific location. Choose automatically & see what it does. If it can't find it, then you will either have to find the disk containing the driver software for the device or download a driver from the manufacturers website.