SP2016 migrated sub site is not browseable.

Question

I have migrated sharepoint 2013 to 2016 using content db mounting process... It works fine but some of the sub sites (not all) is not accessible from the browser throwing 404 file not found errors. If I look at site content structure page those sub sites are available but none of the pages are browseable. How to troubleshoot this issue.. any help is much appreciated.