Nose is collapsed on one side. How can I fix it?

Without seeing the nose and actually examining the nose, diagnosis and treatment options would be impossible. There are many cause for these issues and usually demand strengthening the nasal structural framework with cartilage grafts. You need to consult with an experienced expert in rhinoplasty.

This can have a number of different causes.  The most common is a soft or crooked columella.  A columella strut may be useful, but you will also have to look and make sure the septum is straight and the alar rim is strong enough.  Get a consult with an experienced nose surgeon.  This is a complex problem.