My cousin sold our jointly owned flat in Mumbai without telling me, what can I do?

I'm feeling completely betrayed. My cousin and I co-own a flat in Andheri, Mumbai. I recently discovered he sold it without informing me or getting my consent. We inherited this property from our grandparents and I never thought he would do something like this. I only found out when I got a notice to vacate from the new owner. I tried contacting him but he's avoiding my calls. What legal actions can I take to reclaim my share or stop this sale?