When you change your nameservers to ours after signing up for a shared hosting account, there needs to be time for DNS propagation. While you may see the site as down, it's just that your ISP hasn't update their DNS yet. That can take anywhere from 2-24 hours, so you'll have to give it time.

To verify the site is working you can either test from a different network / ISP, or use the temporary hostname and URL provided to you in your welcome email.