It's hard to give concrete advice here because this is a very personal thing. First: It is quite common to feel empty when such a major goal have been achieved and probably you are left with no further major professional goals now. But face it: You don't need major goals in your life - smaller goals also work fine.
If you lost motivation to do research, I can recommend to try to enjoy doing science for the next time, and not focusing on the outcome too much. As a tenured academic it is one of your privileges to follow your scientific interests and not to worry about the outcome from the beginning. Pick simple problems, and think about them in simple ways. Also, talk to other people about your field and be interested in their work. This will give you insight in other areas but also new perspectives on questions you may already have.
Lastly, and to prove that you are not alone, here is another guy that felt the same. Although I usually don't like it to quote the famous guys on such problems, but it's written well and carries some good advice (but following it will probably not always lead to a Nobel prize as in this story):
Then I had another thought: Physics disgusts me a little bit now, but I used to enjoy doing physics. Why did I enjoy it? I used to play with it. I used to do whatever I felt like doing – it didn't have to do with whether it was important for the development of nuclear physics, but whether it was interesting and amusing for me to play with. […] I'd invent things and play with things for my own entertainment.
So I got this new attitude. Now that I am burned out and I'll never accomplish anything, I've got this nice position at the university teaching classes which I rather enjoy, and just like I read the Arabian Nights for pleasure, I'm going to play with physics, whenever I want to, without worrying about any importance whatsoever.
Within a week I was in the cafeteria and some guy, fooling around, throws a plate in the air. As the plate went up in the air I saw it wobble, and I noticed the red medallion of Cornell on the plate going around. It was pretty obvious to me that the medallion went around faster than the wobbling.
I had nothing to do, so I start to figure out the motion of the rotating plate. I discover that when the angle is very slight, the medallion rotates twice as fast as the wobble rate – two to one. It came out of a complicated equation! Then I thought, "Is there some way I can see in a more fundamental way, by looking at the forces or the dynamics, why it's two to one?"
I don't remember how I did it, but I ultimately worked out what the motion of the mass particles is, and how all the accelerations balance to make it come out two to one.
[…]
I went on to work out equations of wobbles. Then I thought about how electron orbits start to move in relativity. Then there's the Dirac Equation in electrodynamics. And then quantum electrodynamics. And before I knew it (it was a very short time) I was "playing" – working, really – with the same old problem that I loved so much, that I had stopped working on when I went to Los Alamos: my thesis-type problems; all those old-fashioned, wonderful things.
It was effortless. It was easy to play with these things. It was like uncorking a bottle: Everything flowed out effortlessly. I almost tried to resist it! There was no importance to what I was doing, but ultimately there was. The diagrams and the whole business that I got the Nobel Prize for came from that piddling around with the wobbling plate.
From "Surely you're joking, Mr. Feynman", by Richard Feynman
