What does it mean when I am unable to shed tears?

I have been there, usually it is because I am too emotionally tired and so drained of all energy that I just can't even bring myself to cry. Dry crying is horrible.

You are probably feeling numb. When your body doesn't want to be upset about something, you keep it hidden.

Sometimes you just don't have enough power to cry, it doesn't mean that you are not being hurt by this situation. You don't have to cry with oyour eyes to count it as a real cry, sometimes it hurts so bad that you can't even put it out in tears.

Crying is something that makes us feel vulnerable, and you can actually condition yourself to not cry when you may want to in order to retain some order and try to control the situation in some way. Sometimes even when we cry, which is one of the most amazing forms of body language, some people will only get more agitated and, since we subconsciously remember this, we may learn to associate crying with making ourselves weak and only making things worse. I went through a time like this myself, and actually had to take a few years to train myself again that it's ok to cry no matter who you are and it's a natural and healthy way to make ourselves feel better. Crying is pretty amazing, and you need to sit with yourself and consider; are you trying to make yourself look strong? Has anyone made you feel worse when you cried? Are you afraid of showing an image of yourself that isn't perfect? Next time you feel sad, take yourself to a safe and relatively quiet place where you can be alone and keep reminding yourself that you're safe. Just allow things to happen as they happen, and don't try to control the situation.

you may be dehydrated, but it also could be a form of emotional detachment that is causing you to be unable to cry

It means that you have no more tears that need to be shed. It is time to move on and shed away the layer of your life that is making you upset. Fix the issue or just find something to make you better!

Now I am not trying to be funny but it could be a physical issue if you literally have none, just saying. If you can't because you are sad. Well I got that way when the depression got past the "sad stage" and just turned to being numb. For me after being sad and miserable long enough my mind and body told me "enough is enough" and just went into stay numb phase. If this is where you are it is time to look for help. While it may feel good to just pretend you are well or your issues are no longer there, it only delays what WILL happen sooner or later.

Crying is your body's reaction to unhappy or extremely exciting things. When/If a person is 'unable' to cry it may be a deeper problem that should be treated by a professional.

To me, it means that a situation is so overwhelming, I am blocking out the pain. I have found this isn't healthy as pain builds up whether you are blocking it or not. To cope, I will meditate on a blocked pain a little at a time so memories and feelings don't stay repressed. When I'm feeling totally incapable of feelings, I'll "trigger" a laugh or tear with favorite videos or movies.

I have experienced an inability to shed tears when I am taking a certain class of anti-depressant. The medication evens my moods out so that I am never really exuberant, and never really miserable. It's a trade-off, but I'll gladly give up wild joy in order to avoid the depths of despair.