Something went wrong - try your request again.
(The most common problem is maxing out the queries per second allowed by the Guardian API, so your query should work OK next time.)
Alternatively, start over.
(If you're hacking the 'months' parameter, try going back and clicking the '6 more months' link instead as this spreads out the API calls over time.)
If you're still having problems email me at elliott [dot] sp [at] gmail. Thanks.