You probably booted into download mode by mistake. Unless you're flashing something (little blue progress bar increasing from left to right), just hold the power button in (it could take up to 30 seconds, depending on the phone) until it restarts.
If it restarts into download mode again, the Android system is gone. Then tell us the make and model and we can give you links to an image to flash and instructions on how to do it. (But if you just pushed the wrong button, it should restart back to normal.)