I have a special hijab that I wear when I pray. Sometimes my hair in the back pokes out through the fabric, not full length hair strands but almost like half a cm of hair. I do not think that you can see it unless if you watch closely. I wonder whether my prayers are invalid when this happens. If they are invalid, do I have to make up for all my prayers? I have no idea how many prayers that could be. What should I do?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

If an insignificant part of the ʻawrah (body parts that must be covered as per the sharee'ah) is uncovered during the prayer, this does not invalidate the prayer according to the majority of scholars. This is the correct scholarly view in this regard.

Ibn Qudaamah wrote:

“The slight uncovering of the ʻawrah does not nullify the prayer according to Ahmad and Abu Haneefah. Ash-Shaafiʻi, on the other hand, held that the exposure of an insignificant part of the ʻawrah during the prayer nullifies the prayer because the validity of the prayer is conditioned on the proper covering of the ʻawrah; there is no difference between the slight and full uncovering as is the case of looking at it. However, we have as evidence the following narration: It has been narrated on the authority of Ayyoob that ʻAmr ibn Salamah Al-Jarmi said, 'My father went to the Messenger of Allaah, sallallaahu ʻalayhi wa sallam, along with a group of his clan. He (the Prophet), sallallaahu ʻalayhi wa sallam, taught them prayer and said, 'The one of you who is most versed in the Quran should lead you in prayer.' I knew the Quran better than most of them because I had memorized it. They, therefore, put me in front of them, and I would lead them in prayer. I wore a small yellow mantle, which, when I prostrated myself, went up on me, and a woman of the clan said, 'Cover the back side of your reciter from us.' So, they bought an Omani shirt for me, and I have never been so pleased about anything after embracing Islam as I was about that (shirt).' [Abu Daawood and An-Nasaa'i] Moreover, ʻAasim Al-Ahwal reported that ʻAmr ibn Salamah said, 'I used to lead them in prayer with a sheet of cloth on me that was patched and torn. When I prostrated myself, my buttocks were exposed.' These reports were circulated by scholars and such an act was not criticized. It has not been reported that the Prophet, sallallaahu ʻalayhi wa sallam, forbade it. Moreover, when the prayer is declared valid despite the presence of a given invalidator of the prayer to a significant degree in case of a valid excuse, there should be a distinction made between the presence of the invalidator to a significant and insignificant degrees with no excuse, like the case with walking during the prayer. In addition, it is difficult to avoid slight exposure of the ʻawrah; it is pardoned just as is the case of the emission of insignificant amount of blood. If this view is established, then the limit of the significant part of the ʻawrah exposed is what is considered significant...” [Al-Mughni]

Ibn Taymiyyah wrote, “If an insignificant part of the woman's hair or body was uncovered during the prayer, she is not obliged to repeat the prayer and it is valid according to most of the scholars. This is the view adopted by Abu Haneefah and Ahmad.”

Based on that, your prayer is valid and you do not have to repeat it.

Allaah knows best.