This page sums up request limits pretty well.

Basically:

• Don't make more than 30 requests per second
• Without a key you can make 300 requests per day, with a key you can make 10,000 requests per day
• With an access token, the 10,000 request limit is just for your app for that user. *
• Without an access token, the 10,000 request limit is shared between apps on the IP Address.
• You must obey the backoff field which is in the wrapper object, if the field is there then you must wait the number of seconds it contains before you make an identical request.
• Making an indentical request more than once a minute is unlikely to return new results

( So if a user/server has mutliple apps using an access token (authenticated), each app can make 10,000 requests per day. Each user has up to 5 blocks of 10,000 requests, but a user can have more than 5 authenticate apps running at once)*

It looks as though you arent passing a key, so I would imagine that your problem is that you've gone over the 300 requests limit. To get a key just register your application here

This page sums up request limits pretty well.

Basically:

• Don't make more than 30 requests per second
• Without a key you can make 300 requests per day, with a key you can make 10,000 requests per day
• With an access token, the 10,000 request limit is just for your app for that user. *
• Without an access token, the 10,000 request limit is shared between apps on the IP Address.
• You must obey the backoff field which is in the wrapper object, if the field is there then you must wait the number of seconds it contains before you make an identical request.
• Making an indentical request more than once a minute is unlikely to return new results

( So if a user/server has mutliple apps using an access token (authenticated), each app can make 10,000 requests per day. Each user has up to 5 blocks of 10,000 requests, but a user can have more than 5 authenticate apps running at once)*

It looks as though you arent passing a key, so I would imagine that your problem is that you've gone over the 300 requests limit. To get a key just register your application here