HI Amit,
Please add the below outputs
show version
show virtual-chassis
If the new member is member 2 please try to login to the member from master VC
reqeust session member 2
ELSE
Try pinging the member from shell
ping -Ji fpc2
If both works and members is still showing inactive possibly it has a different Junos compared to master and Backup.
How this can happen as this was working fine for last 7 months?
May be it rebooted and booted up from alternate partition.
Thanks
Partha