Can "unknown error 21" be fixed ?? (iphone 4)

Recently my iPhone 4 just reset itself out of nowhere and wouldn't turn back on. I placed it on a dock & the connect to itunes screen showed up. I plugged it into my computer and when it began to restore it stopped and the unknown error 21 window popped up. This has occured on 3 different computers & now I'm unsure how to handle this.

-Please help.