First, if you have a customer in front of you, take care of them and get them on their way.
If you have to voice authorize a card, use the phone number on the phonelist page for Credit Card Support.

Next, check your internet connection by logging into your paystub or email.
If that worked then your internet is functional and we need to look at your datawire box.

The datawire should have the link light on as well as D4.
(If link is not on, the network cable is unplugged from the back.)
Reset the datawire box by removing the power cord from the rear and replacing it 10 seconds later.

The status light will flash, it should be solid within 20 seconds if everything is working.
If at this time you can run a card, try it and watch the light under D4. It should come on if the register is actually sending the info to the datawire.

If this light did not come on, then reboot the main register.