Web site access

Having problems accessing company web site via DSL connection. A search for the site finds two links, which when clicked, IE says cannot display page. Going outside company on dial up lines, the web site can be accessed. Checked DNS at both website and internal servers and unable to find any reason for the non-access. Restarted the company main DNS server and was able to access site previous night. However, this morning, site again inaccessible. Contacted both website host and ISP, who are unable to determine any reason for issue. Any comments? Dumping a lot of points for this one.