You reset the SMC to resolve the problem. It happens; no need to worry unless it happens constantly, which would suggest hardware failure. It can also happen if you run the battery down to 0% before you plug it in. (Never do that.)
Mac mini (Mid 2011) 2.7 GHz i7, 8GB RAM, Crucial M4 256GB SSD + 500GB + 1TB FW800 OWC Mercury Elite Pro mini
iPhone 4S • iPod nano 8GB • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"