Without a registered API key, your server's IP address will be restricted to 300 calls per day300 calls per day. With an API key, that limit is increased to 10000 per day. What most likely happened was that you exceeded the (free) 300 quota.
After you registerregister for an 2.0 API key, change your request URL as follows:
https://api.stackexchange.com/2.0/questions?key=YOUR_API_KEY&pagesize=50&site=stackoverflow&tagged=xpages&order=desc&sort=creation&page=1