Sharepoint 2010 Visual Web Parts getting removed from the page

In our organisation we have a  lot ofSharepoint 2010 Application Pages . After SP 2 Upgrade we found that some of the visual webparts getting removed from the pages. and we could able to add some visual webparts into the page , and some webparts are not getting loaded even if we try to add again saying this type could not be found or it is not registered as safe . 

This issue occuring after upgrading to SP2

Thanks

Sundar