Orders not processing, nothing has changed on my admin!!

I went live with my ZenCart a week ago and everything has been running great. Two days ago orders stopped coming in (very odd) and now I have customers calling in saying there is a prompt coming up saying "This transaction cannot be processed - your card could not be ran, please check all information or call us for assistance" every time they try to send their order through! I even tried to buy something off our ZenCart knowing everything was correct and it gave me that error message! Nothing has changed on my admin side - this just happened all of a sudden. Help! I'm losing orders!

-Andrea

You need to provide much more information
What credit card processor are you using for a start
What ZenCart version?
Other details as requested when you start a thread