Windows 2000 Server Backup

My Windows 2000 server backups start running and then stop, and I get a message saying ” Add DLT media to free media pool or to media pool \backup\dlt”. The first week I setup my backups they ran fine, the second time around they start and then stopand I get this error each day. When I go to my media pool “backup\dlt”, all the media is there. Can someone help me please!