Take a look at this : Clickjack protection for Non-Setup Salesforce Pages.
If you had run your critical update by mistake it could be one of the reasons why you are seeing blank pages.
This critical update enables clickjack protection for all non-setup
Salesforce pages to protect against user interface redress attacks.
The article in the above reference specifically talks about this issue:
If your organization displays non-setup Salesforce pages within a
frame or , it’s possible that the pages will either display as
a blank page or without the frame after clickjack protection is
enabled. The behavior varies depending on your browser and its
version. Although there are reasons to frame pages, framed pages can
be used by hackers.
Also the clickjack protection cannot be disabled without contacting support :
In the Spring ’14 release, the ability to disable clickjack protection
from the Session Settings page will be removed, so you’ll have to
contact salesforce.com Customer Support if you need to disable it.
However, disabling clickjack protection is not recommended.
Read more here to know what and how it impacts you ( though much of this was discussed way before this was enabled full scale)
Upcoming "clickjacking" protection
This may be a better guide :
Winter 13 Clickjacking protection - is the following expected or a bug?