Welcome to the Bunnings Workshop community @Vishal0405. It's great to have you join us, and many thanks for your question about repairing a broken fence.
That appears to be one of the rails that has snapped. If there is no apparent reason why the rail has broken, then it's likely there was a compromised section of timber in it that has fractured. You'd be looking at removing all the pailings for that section of fence and replacing the rail. Once you've removed the pailings, you can pick up this step-by-step guide at the Installing rails and plinths for a pailing fence step.
It doesn't look like a particularly old fence, was it installed recently? If this is a new fence, I recommend speaking with the installer about this issue.
Please let me know if you have any questions.
Let me also extend a very warm welcome to the community @Vishal0405. We trust you'll get heaps of helpful advice and inspiration for all your projects around the house and garden from our wonderful members.
Please let us know if you have any follow-up questions from Mitch's reply. We're here to help.