No More Printers from the printer folder

Hi,
Pleae help me out with this probllem.
We have NT4.0 and 30 users. Some of the printers are with Jet direct and some of them are sharing priners.
This is problem occurs, when we try to print from any application like work or excel, the printers won’t respond. When i try to check the printer from the printer folder. All the network printers are disappered. I need to reboot the computer to make to print. Please help me out
Thanks,
suresh