FIrst try to rule out problems such as teething, ear infection or other medial conditions. A lot of the time, these medical issues and illnesses only manifest at night and I would be suspicious if this came totally out of the blue.
If you rule out illness or teething then this may just be a phase. the experts always say that right before a period of developmental growth there is often a regressive period where the child seems to slip backward. Only you know when you've crossed the line btw a passing phase of insecurity that you can suffer through and a real problem with sleep habits. Try spending a lot of time with her during the day, don't push her to be a "big girl" and indulge if you can some of her regressive behavior.
If it keeps up then you have to decide on one of the sleep training approaches and stick with it. there have been many discussions of these issues in this forum and I'm sure just reading through some of those dialogues will give you an idea of the different schools of thought on this and where to get more information.
GOod luck!!