First, can you try update your Brave to the latest release 0.66.100, @tushar? And see if it can solve your issue.
You only mentioned about pinned tab. Did normal tab also gone after restart? What is your “On startup” setting under Brave’s Settings?
Also, steps to reproduce will be really helpful to help “diagnose” your issue. Because pinned tab is attached to a single window. So if you close that window, it’ll also “remove” the pinned tab.