If you have a dynamic IP, resetting your modem might work.
You can also look here:
Realm Down & Temp Ban Reversal Guide
Tiffiter's Unabridged Guide To Bypassing a Temp IP Ban

First of all I recommend just waiting it out, as long as you don't keep checking frequently, aka, faster than 15 minutes, it should be gone in a jiffy.

But...
How to get rid of Realm Down
1 Go to Start.
2 Click on 'Run'
3 Type in 'cmd' and press enter.
4 Type in 'ipconfig /release'.
5 Type in 'ipconfig /renew

This depends on if you have a router/type of connection though.

This advice is sound. Be patient, and it will probably go away in a jiffy.

It's caused by changing characters too quickly (Blizz sees this as suspicious I guess)

Constantly checking/failed log-in attempts doesn't extend the time restricted

The time is set by blizzard upon them issuing the temp rest/realm down

Creating/joining to many games in a period of time (20 per hour is the general idea) & switching screens/character changes can also cause it

There's a link in my profile to Blizzard Techsupports explination of TR's

Sorry I can't link from my phone