Backup Exec 2012 cannot detect the Dell TL 2000 Media Changer

Dear All, 

I owned a backup exec 3600 appliance and have configured a tape out to Dell TL2000 tape library. 

recently the BE reported that one of the tape drive malfunctioned. and I got dell to replace the problematic drive. 

unfortunately the problem worsen ... now even the whole tape labrary (media changer) cannot be detected. 

I got the whole chasis and both tape drive replaced and still cannot detect the media changer ? 

what might be the issues? 

my backup exec is Backup Exec 2012 revision 1798