What happens if you press the eject button (The 'E' under the drive selection box - or right click the selection box and pick Eject) in ImgBurn? Any error message?
Normally, if the tray won't open, it's because it's been locked by software. Turning the power off and on would usually reset that.
If after doing that, the button on the device itself still doesn't work, maybe it's just faulty.
What do you have providing power to the device? Hopefully it isn't just being powered by a single usb lead.