503 is usually the minecraft multi-player session server being temperamental. Check Mojang | Support Center
to see what is going on.
Currently playing: ESO
Currently retired from playing: EQ2, Vanguard Saga of Heroes, Fallen Earth, WoW, Mortal Online, Perpetuum, Rift, SWTOR, EVE (again), Minecraft, Archeage, Elite Dangerous, Landmark (can you retire from something not released?)