Before resetting the printer to it's factory condition you should first attempt these troubleshooting steps:
Check to make sure that the ethernet cable is firmly in place.
Check for paper jams.
After resolving any of the physical issues mentioned above, power the printer off and on again to clear any error lights the printer might display.
Refresh the printer list from within Register after troubleshooting physical issues (help doc: https://zing.zendesk.com/hc/en-us/articles/203250204-Locate-receipt-printers-in-Register).
To perform the factory reset, perform the following:
Turn the printer off. Hold down the feed button, and while continuing to hold the feed button down, turn the power back on. Continue holding the feed button until the printer prints out a series of test receipts - the second of which will include Network Configuration information.
The IP Address field should give you a number besides 0.0.0.0 (you will not be able to connect to 0.0.0.0), and unless you have intentionally configured it otherwise, DHCP should be enabled.
If DHCP is not enabled (DHCP allows the printer to automatically obtain an IP address), then you can enable DHCP by resetting the printer to it's default factory condition.
To reset the printer to it's default factory condition, perform the following steps:
UNPLUG THE PRINTER (always unplug all electronic devices before attempting to work on them in any manner outside of normal use).
With a phillips head screwdriver, remove the small panel on the bottom of the printer where you will find 4 dip-switches. The dip-switches will likely all be in the 'up' position (this is the position that they should be in for normal operation).
Set dip-switch 4 to the down position (dip-switch 4 is the dip-switch farthest to the right - you will be able to determine the proper orientation by reading the text on the bottom of the printer - the text shouldn't be upside-down).
Plug the printer back in, hold down the feed button, and power the printer on until another set of test receipts are printed (the second receipt should include your newly acquired IP address).
Now that you have a valid IP address, power the printer off, UNPLUG THE PRINTER, set the fourth dip-switch to the 'up' position again, and replace the dip-switch cover.
Plug the printer back in, power the printer on and attempt to reconnect to the printer within Zing - this time you should be able to find it.