Hold the power button down until the phone restarts (about 20-30 seconds). If it's just on the "Volume Up to download, Volume Down to Restart" screen, just hit the Volume Down button. If it restarts in download mode again, you need to reflash the stock ROM (see [Samsung] How to flash Stock ROM via ODIN) and I hope everything is backed up, because you''ll lose it all. (No, there's no way to fix it without losing everything.)