Access web server from internal network

My company is currently running a web server (iis 5.0). The public can access it from the internet and employees can access it from the intranet. We have a firewall which is a cisco pix which also runs NAT. The web server is on the private side of the network but whenever the isp goes down the private side of the network cannot reach the web server. The ip address scheme in NAT seems to be correct( public to private). Thanks.