This sounds like what happened to me on my honeymoon!!!
After you ejaculate, there is a recovery period. It takes time for the system (prostate/testicles) to re-create semen.
When you are younger, the recovery time is quick, but as you get older, it takes longer. Thats why younger guys can ejaculate as quickly as a couple of hours right after the first orgasm.
If nothing comes out when you masturbate, it simply means you have not given your system enough time to reproduce seminal fluid.
You will still enjoy the feeling of the orgasm, but there is nothing there to come out.
Oh, another thing. If you want to keep the prostate in good shape, take some zinc. This will help the prostate stay healthier as the prostrate thrives on zinc.
Hope this helps...