Don't worry, most back panels of android devices are designed to be removable. Line up the panel that fell off with the back of your tablet and look for tabs and slots that can either slide or snap into place. Generally one or two side will slide into place while the rest will click in. If it seems to need more than gentle pressure to go in, stop and try a different side first. If you still have your device's manual(sometimes these can be found online, try googling your device name and "user manual") it may contain more detailed instructions for this.