I have experienced the same with my daughter. She has slept through the night since she was 2 and half months old. I have read that there are several reasons for children to start waking in the night. I found that the main reason for my daughter's change in sleep pattern was due to her molars pushing through. After they were in she started sleeping through the night again. Now she is 19 months and tends to wake up at least once. My doctor told me that she is probably having more active dreams and some of them may not be pleasant. She advised me to let her cry for about 5 minutes- go in and give her assurance that I am around and then leave (bring water or ask the child if they would like some water). For the most part, she tends to fall back to sleep. I used to wait for her to fall asleep and then leave, but I have now gotten into the habit of giving her a kiss, telling her I love her, and then tell her I will see her in the morning.
It is really hard to have a crying child at night- especially when everyone else is trying to sleep. I had to show my husband literature and battle his inability to let her cry. It took me several weeks to get him on board, and to have my daughter understand that bedtime is bedtime.
It is so difficult to hear our kids cry, but they are safe and learning important habits. There are a few things that are non-negotiable...car seat use, going to any kind of child care (if used), and bedtime. Usually giving a choice to kids is great- except when there is no choice to be had.
You are not alone...:)