As you say, this can mean that your IP address has been blocked. I'd make sure that you specify the key parameter on 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've set up IP Address Restrictions within the Developer Console, you've allowed the correct IP address, just click the project within the list and you'll see the allowed IPs.

If you're still running into issues, you might want to look into printing out the values of the status
and error_message
elements from the response from Google, you'll 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 doesn't mention an IP address restriction, it may well give you enough information about the problem to Google a fix.