it happens to me sometimes, if i switch chars and acc too fast, mostly while muling...
after 25!!!! min, it usually works, sometimes, if it pisses me very off!, i have to wait 30min... thx btlnet and blizz,

edit: right... start worrying, if you cant play after 30 min, or an hour!
if u did something bad (rofl), than it could "ban" u for a day, i heard about this from friends... if blizz MIGHT think, that u COULD do sth to ahrm them/the "play-economy" they ban u for a day, if they cant be SURE u did sth "wrong"...
blabla