I am wondering to know the once-beautiful meaning where I could not find its meaning in the dictionary or internet!
The once-beautiful building was now dirty, decrepit, and roofless.
It simply means the building used to be beautiful previously.
at some time in the past; formerly.
So, "once-beautiful" means the building was beautiful previously and stands in contrast to its current state - "dirty, decrepit, and roofless".
You can add "once" to almost any adjective, such as "once-important".