Something like iExplorer might help you jar it out of that, maybe....been a while since I used it but it can toggle into out of DFU mode or something like that....
More likely you have a hardware problem. Might need to open it up and disconnect the screen entirely and see what it does afterwards. (Opening the phone and disconnecting the battery is also a proven method to power down when buttons aren't responsive!)
Some photos of your screen might help determine if you have a hardware issue.
There is a very slight chance there is a software issue related to the screen. By any chance is the LCD/Digitizer in your phone a 3rd party replacement? I went around in circles on replacing a screen on my 4S once and I eventually discovered that the screens can actually require particularly firmwares on the phone to work properly.
My instinct on this one says your screen and digitizer has physically failed. They are somewhat independent, so even if the image isn't displaying right the fact that you can't swipe to turn off suggests the touchscreen is bad as well.
