You should be able to run this command as Trevor points out above.
Convert-SPWebApplication -Identity http://yourWebApplication -To Claims -From Legacy -RetainPermissions -Force
Also when migrating the databases it is important to test for errors first before running the mount command:
Test-SPContentDatabase -Name WSS_yourWebApplicationContentDB -WebApplication http://yourWebApplication
Mount-SPContentDatabase -Name WSS_yourWebApplicationContentDB -WebApplication http://yourWebApplication
Otherwise the only account that will end up getting access is your farm admin account in SP 2010 AD was the default Web Application Authentication, in 2013 MS switched to Claims Authentication as the default for Web Applications (This is how I recognized the issue when I was migrating from 2010 to 2013 the first time).