I have heard of some issues about their server being overloaded, so I would advise waiting and possibly rebooting your device before you try anything extensive, as some solutions can be very time intensive, as you try to find the ONE thing that is wrong, ahh but that is computers for you! Hopefully that is all it is, but if not I am sure that we will all try to help you figure this out! Just be patient! Don't give up, because there is always a solution!
This whole Cydia coming up blank issue has happened to me quite a few times now.
And while it has now happened to me enough times to have found various other ways of bringing back the sources and packages list that is a lot easier and less drastic than restore\re-jailbreaking I still agree that it is less than acceptable in terms of expectations should be for a software program that makes its self a necessity for the proper operation of a jailbroken iphone. When the fact that when any of the various means I have found thus far of bringing the content display back up are used a great many of the repos I had spent many hours of my time adding are no longer there and must once again be added by hand all over again in order to access them.
So far as I can tell this behavior of Cydia revolves around cracked apps for jailbroken phones which the developers have posted for sale. (eg. FolderEnhancer etc.)
I'm no programmer or anything but it seems that somehow whenever a cracked version is discovered a bit of code or something goes to work and plays havoc with your phone and or Cydia.
I'm not claiming to be right on the money correct on that but I know I'm in the ballpark. And having said that and without launching into a long diatribe and resulting disscussion revolving around semantics I would just like to toss out my opinion that a practice such as mentioned is the action of hypocrite. And I say that politely.