how do i change e-mail viewing to html in outlook 20007 ?

Starting today my incoming e-mails changed to what appears to plain text
including no header information in the list of e-mails and when I open it. I
checked the "Trust Center" setting for "e-mail security" and the "Read as
plain text" box is un-checked. All the e-mails are from known sources and I
use Network Solutions for e-mail service.

Your help is appreciated.

Thanks
John