Siamese cat lost (Gresham)

She was my first kitten that i've ever had, i got her a little over 2 years ago and only let her outside on a harness while i was around. whenever she ran outside she never went far and always came before nightfall. last week i came after staying with my grandparents and i find out my brother unexpectedly locked her outside for the weekend while they went hunting, she was waiting on the couch for someone to come . i'm really worried something happened to her, i've been searching since she went missing and no one around our neighborhood has seen her either... she has a collar with my moms phone number on it. she was basically my baby