0. Connect AC adaptor, make sure it's plugged in
1. Check if screen brightness is not set to 0 - AFAIR it will be F1-F2 keys
2. Connect USB device like mouse/keyboard/pendrive - it can wake up
3. If 1&2 doesn't help - you can shut laptop down by pressing power key - then start it up and check Energy Saver settings
4. If it will stuck in sleep again consider resseting PMU, read Apple Support article - http://support.apple.com/kb/HT3964
5. If PMU reset won't help contact Apple - it can be kind of hardware problem