Sometimes you have to do some more zigzagging to get it to work again. I use ardweenys a LOT here for their low cost, and I find this problem somewhat frequently when I'm in a hurry. After I hit upload, I tend to turn on the serial monitor. (which always turns off and closes when you upload??) When the upload finishes, it reboots the ardweeny. But when you turn on the serial monitor, it reboots it AGAIN. If these reboots happen too close to each other, (turn on the serial monitor within 1 sec of the upload finishing because you are impatient) it often causes the serial port driver to jam up and you get this message when you try to upload.
Sometimes trying again fixes it. Sometimes quitting the IDE and relaunching fixes it. Sometimes it just plain gets STUBBORN and you have to get more drastic. I was just rebooting my computer, and that always worked, but I've found a faster way. Open your serial select and note which one is checked. Quit the IDE. Unplug your serial adapter. Relaunch the IDE. Select a serial. Your serial interface will not be listed, nothing will be checked. Select something else. (may need to select something very different, mine is a /dev/cu, so I select a /dev/ty) and try to upload. It will fail. Quit the IDE. plug back in your adapter. Relaunch IDE. Re-select your serial adapter. Should work now.
(If not, you are probably going to have to reboot your computer to reset the serial driver)
And to avoid the problem, avoid rebooting your arduino over serial WHILE it is already in the process of rebooting.
(this could also be caused by hitting upload a second time while an upload is in progress, I haven't tested it that heavily)