You've dislodged the tamper switch, needs to be re seated.
Don't know your panel, but in general it should go something like this.
Open / Close the control panel - enter your code, press reset button if fitted, or a version of.
Please be aware that there is a mains supply inside the panel, might be best to turn off its supply until you've done.
As an aside, if the alarm sounds when you switch off the mains, you'll need a new battery as well.
acorn, April 2011
