DHCP server does not release ip and is not found by clients

I have 300+ machines on my network. The problem is when my staff tries to log back in, they were unable to get an IP address. Checking on the AD server which I used to serve as a DHCP, it was up and running actively.
But running the ipconfig command and trying to renew the IP failed and it was unable to connect to the DHCP server. I tried everything and restarted the DHCP Client and Server on my AD, but I have no luck.
What else should I do?
