An inability to cry can be a symptom of depression or stress. For some reason the body's emotional response can be to prevent you crying - though God knows why.
I used to be able to cry easily, which was a great release, but then I went on anti-depressants and couldn't cry - even when I wanted to.
I've been off them for four years but still find it difficult to cry. I wonder now whether it's just that it's a symptom of getting a bit older (well, in my 30s!) and that I'm emotionally a bit tougher than I used to be.
Probably most important not to worry about it - bodies are daft things and while it's weird not being able to cry, maybe it's just your body's reaction to what's going on.
One advantage for me now is that situations that would have had me blubbing in public (such as at work) don't have that effect, even though I'm 'crying' inside. It makes me feel stronger now and better able to cope - and therefore I feel less like crying.
I know it's not a conclusive answer, but I hope it helps to know you're not alone and that there is nothing technically 'wrong' with you.