DHCP expiry problems

[McLarnon]

I came into the office this morning and various PC's on my network had lost their IP addresses. When I accessed the DHCP manager on my server (NT Server 4.0) I can see all the PC's that have lost their address, greyed out. When I select any of these PC's it is showing their lease has expired but I cannot see how to renew the lease (if that is possible). I now see that other PC's on the network lease's are about to expire at various times over the next 48 hours.
On some of the PCs that addresses have expired I have made their address static so that they can be used, it works short term but I need to sort this quickly. Does anyone know how to sort this?

Thanks

McLarnon