Can you, from within your network, ping your website by name, i.e. www.mycompany.com?
When you do the ping, does the command return the correct IP address for the site?
If you ping from outside your network, do the IP addresses match with your internal results?
If you do not have ready access to an outside machine from your office, you can also use on of the DNS sites, like www.dnsgoodies.com to do the DNS lookups for you, or use nslookup pointing to an outside DNS server.
I suspect that the hosting company may have changed your public IP, either inadvertantly, or after sending a notice to the contact at your company who may find it in a spam folder, may now be no longer associated with your company, or just neglected to mention it <g>. If this is the case, you will need to edit your internal DNS record for the site Then, I'd probably get on the phone with the hosting company to determine what happened, and why were you not notified.
Further to the above expert advice strictly speaking an external site should not need a specific entry in your local DNS servers as they should forward any unknown requests to external DNS by default. The only time it may become needed is if you host a subdomain of the website internally and the main part externally. The best practice for this though would be to make entries for any local sub domains in the local DNS and leave the main site to be looked up via the correct method.