In a limbo stage in PhD. How to get out of it?
Scientific work is always a high stakes gamble. You are never guaranteed to succeed and several failures in a row usually result in a quite depressing state when you lose the willingness to try anymore completely. It is nothing unusual. Here are the techniques that I usually employ when getting nowhere for a long time.

1) Step back and relax. Forget about it all for a month at least. Live your life (I hope there are some things you enjoy besides solving problems, working in the lab, and writing papers).

2) Talk to people in your field or go online (to MathOverflow, say) to see what they are working on and have trouble with. If you are not at the very bottom, there will always be a lot of people whom you can help. Boosts your self-confidence quite a bit.

3) Remember that you are under no obligation ("obligation" is not the same as "pressure") to produce anything or to prove to anyone that you are any good. You came to the parallel universe to wonder, explore, and play. So, just do exactly that. Read something funny in your field, ask yourself some "stupid" questions, etc. Just get the feeling that you belong to the scenery around you and can enjoy small streams and hills before trying to cross a big river or to climb a mountain.

4) Look at Hubble telescope images and try to imagine all the distances, times, and energies involved. That puts you with all your worries, ambitions, and desires in the right perspective. Don't care too much about the problems of something as insignificant as one unit in over 7 billion on some God's forgotten planet in the far corner of some minor galaxy, totally dispensable and replaceable any time. Concentrate on the outside world. You can figure out something about it before your time is over, that is the only game really worth playing in serious, and you are totally qualified to participate in it despite the fact that there always will be much stronger players around. 

There are more, but these four work well enough. And, of course, complain about your situation to your friends any time you want. No need to handle everything alone :-)