Have you updated the iPad's iOS recently? iOS 4 (I think) reduced the allowable current draw on the connector port, which made some older peripherals stop working. It is also possible that the dock/speakers may have developed a short or other minor problem.
If you have not done so, try restarting and/or reseting the iPad.
It probably wouldn't hurt to unplug the dock for a while either.
That's about all I can think of.
Good luck.
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Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.