Print server disconnect users

Can anyone help me out? I administered over 50 clients on the 2000 Adv. server network, with a printserver running Nt 4.0 server o/s hosting 3 network printers namely hp 4100, and two 4000 pcl series, one on each floor of my office.of recent the print server disconnect few users and they are unable to print until I either refresh the print server by rebooting it, or delete the printer from clents end and re-install which is burdensome everyday, of recent, the print server has refused any of the printer to print complaining of no “valid licence” What do I do? should I remove all printers drivers and re-install,or has it to do with Ip problem? or any conflict with NT server? Please, need your assitance!