Google Calendar error message

I am trying to add a google calendar to my web site, i have double checked the API and Calendar but get this error message, any help would be very much appreciated

While trying to retrieve events, Google returned an error:

{
“error”: {
“code”: 403,
“message”: “Requests from referer \u003cempty\u003e are blocked.”,
“errors”: [
{
“message”: “Requests from referer \u003cempty\u003e are blocked.”,
“domain”: “global”,
“reason”: “forbidden”
}
],
“status”: “PERMISSION_DENIED”
}
}

Please ensure that both your Google Calendar ID and API Key are valid and that the Google Calendar you want to display is public.
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