Welcome to Android Central! Are you on a homescreen? Do you see any apps along the bottom, with a circular icon with dots in the middle?
Can you reboot? Power the phone off by pressing and holding the Power button until the options come up, and select Power Off. If this doesn't happen, then just press and hold the Power button for about 10 seconds until the phone shuts off. Wait a few seconds, then press and hold the Power button again until it turns on.