ABitWrong if you feel up to it then give us a bit more information so that people know how best to advise.
For example, are you on ads as well? What is distressing you/making you cry?
Just to answer you initial question, the diazepam won't physically stop you crying. I am assuming that you have been prescribed them short term to cope with depression/stress/anxiety/some sort of shock or bereavement.
A small dose say 2-5 mg will just take the edge off things iykwim. They may make you less likely to cry, probably make you feel a little bit floaty or sleepy. A tiny bit like having a small amount of wine in the day?