We would need a lot more info in order to help you--like what BYOND version you're running, what OS, etc. The users who've had blank window issues with the pager have tended to be on Windows 8.1, or have needed to reinstall IE because something was broken in their installation. I would recommend you update the .NET framework to the very latest version, and reinstall IE. Updating Flash for IE is also a good idea, although it might not be related to this issue. Also if the system has been running for a while, you could try rebooting.

You could also try a system restore to a previous day where it worked, and find out which versions of the .NET framework, IE, etc. were all in place. If this happened spontaneously then the main culprits are an automatic update gone wrong, malware, or possibly something corrupted on disk. If an update was to blame, then after a system restore you could try applying updates manually one at a time instead of automatically, keeping track of which one you're installing each time, and test BYOND each time along the way to make sure it still works.