As you say, this may mean that your IP address has been blocked. I would make sure that you specify the key parameter in the query string for the geocoding API request.
https://maps.googleapis.com/maps/api/geocode/xml?sensor=false&address=Placename&key=XXxxxXXxXxxxxXXxx
Also make sure that if you set the IP address limits in the developer console, you specified the correct IP address, just click on the project in the list and you will see the allowed IP addresses.

If you still run into problems, you might want to print the values of the status and error_message from the Google answer, you will see something like this:
REQUEST_DENIED - This IP, site or mobile application is not authorized to use this API key. Request received from IP address 123.4.5.678, with empty referer
If it does not mention the IP address restriction, it can give you enough information about the problem to fix it on Google.
Luke Alderton Jan 24 '18 at 11:42 2018-01-24 11:42
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