I dont know what equipment you use or your knowledge of computers, so I will just explain in general terms
Depending upon how you connect to the internet this can be an easy solution. If you have cable internet or dial up, disconnect your connection and re-connect. More often this will require physically powering down the cable modem. The idea is to get a new IP address. Most often just doing a DHCP renewal will not fix the problem as you will most likely get the same IP (although it is a much simpler and quicker method to try first). However you go about this, once you have a different IP address you will be able to log in again. The ban is on your IP, not your account.
I got these before and realised this was the case after I would get banned at home, be able to play at work, then get back home to find I am still banned. Sadly I have a static IP at home so I just have to wait it out.
As for doing anythig bad? Well most likely you did not, except Blizzard felt you did

. My assumption is you were probably doing something like Tristruns or something similar to the effect you were entering and leaving games too quickly. Rare, but does happen to some, is that you can get one of these awful bans for absolutelty no reason at all. A while ago after being in a game for some time doing quests, I switched characters to do more quests. The new game I tried to make timed out, then I got immediately logged out of Bnet. When I tried to log in again I was temp banned for 15 hours. Why did this happen? Who knows, and really who cares since there is nothing we can do about it, Blizzard could care less about our complaints on this.
So the thing to do is this, I have found it now works for me. If you are going to jump in and out of games, give your char a few seconds of non-activity then exit. I usually let the char sit for about 30 seconds then exit. Since I have not received one temp ban