Firstly: The Ruling of Spending the Night at Muzdalifah
Spending the night at Muzdalifah is one of the wajib acts of Hajj. This is the position of the majority: the Malikis, Shafi`is (according to the soundest position), Hanbalis, and a group of the Salaf.
Secondly: The Ruling of Missing the Wajib Act of Spending the Night at Muzdalifah
If one misses the wajib act of spending the night at Muzdalifah, their Hajj is valid. However, one has to expiate with blood unless one’s non-performance was due to an excuse  Amongst the frequent excuses in recent times is traffic standstills due to crowds. , in which case nothing is due. This is by agreement of the four schools of jurisprudence.