Sounds like you may have installed a Cydia tweak which is not compatible with the iPad (likely designed for iPhone only) or is not compatible with 5.1.1
Either way, the easiest way to get things working again is to use safe mode.
Hold down both power and home buttons until the iPad resets. As soon as you see the screen go black and then the Apple logo appear again, let go of the buttons and hold down the VOLUME UP key until the iPad finishes booting. This will put it into safe mode by disabling all your jailbreak apps / tweaks. In this mode you can still open Cydia. Remove whatever tweaks you installed most recently, since these are the likely culprits.
Once you have removed them, reset the device again and let it reboot properly. Keep removing tweaks until you find the culprit or your iPad boots successfully. Always work backwards from the thing you installed most recently, and check the descriptions in Cydia to see if it specifically states it is for the iPad / 5.1.1. If not, and you don't see any details about the tweak on this forum, there is a good chance it is not for the iPad.
Hope that helps. Good luck.