In this weeks Parsha, Bnai Yisrael call out to Hashem for help in the end of the second Perek. Why didn't they call to Hashem before? They were slaves for so long and they didn't call out to their God for help?
Ramban says that Bnai Yisrael didn't know they were slaves before. Paroh had tricked them by telling them that the work they were doing was work each Egyptian had to do at their time. Paroh did this for two reasons: To make a nation of slaves for Egypt and to lower the birth rate of Bnai Yisrael by making the men tired when they finished. This is why one of the two Jews who were fighting asked whether Moshe was going to kill them as he did the Mitzri. This shows a slave mentality; they saw themselves as nothing more than what they were, and Moshe's killing of the Mitzri was nothing special.
Now our question becomes why Bnai Yisrael called out to Hashem at all. In a slave mentality, they should not have been calling out at all. In 2:23 of Shemot it says that Paroh died. Ramban says that Bnai Yisrael were given more work from the new Paroh. This made Bnai Yisrael realize that they should be treated better and they called out to Hashem. Rabbi Mark Smilowitz says that their was chaos when Paroh died. This chaos gave Bnai Yisrael a taste of freedom and made them cry out to Hashem to save them.