My apple magic keyboard keeps un-pairing with my computer.

My mom got the new iMac a while ago and she didnt want the magic mouse and keyboard so she gave them to me. I'v had them for several months with no problems and I'v become very fond of them for gameing. My out of town friend was in town yesterday and staying at his grandmothers house so i wnet over there with my macbook pro 13 inch, i did not bring my magic mouse or keyboard. When i came home i went to go play on my computor and i tryed typeing in my password on my magic keyboard but nothing showed up so i tried again, thats when my mom yelled from the other room that i was typeing on her computer. she turned her computer off and than i re-paired the keyboard with my computor and it worked fine for the rest of the night. When i got up this morning my mom asked me to carry her computer downstairs, i did and when i went up stairs to use my laptop i tryed to type my password in and it didnt work. i checked in system prefrences under the keyboard and it says it isnt paired with my computer. She is the middle of a very big project for work and id like to fix this problem.

Thanks for who ever answers my question in advance.