You can try removing the battery for a few minutes to completely reboot it.
If that doesn't help and if you don't have anything important stored on it you could try a factory reset (aka "recovery", "restore", etc). The support site linked above can probably tell you how to do a factory reset. Keep in mind that you will lose EVERYTHING on it...apps, data, messages, contacts, etc. Usually the procedure is to hold the Volume Up button while pressing the power button, and keep holding Volume Up until it boots into recovery, then use Volume Up/Down to navigate and another button to select the option to wipe everything.