Thank you very much for your question. It is our pleasure to answer it.

In the name of Allah, We praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.

Covering `Awrah is one of the necessary conditions for the validity of the prayer. In prayer, the whole body of the woman except her face and wrists is from `Awrah. This is the prevalent opinion of Muslim scholars. Therefore, a woman has to cover her hair in the prayer. The Prophet (peace be upon him) said: "Allah does not accept the prayer of a pubescent woman unless she is wearing a head covering (Khimar)." (Abu Dawud)

If a woman covers her whole hair so that no hair comes to view then her prayer is valid; tying hair is not necessary. However, if her whole hair or some of it gets exposed then she should repeat her prayer. 

I hope this answers your question.