My iPhone 11 keeps on shutting down

my iPhone 11 keeps on shutting down, and I don't know why, it's just sitting on my desk, till suddenly it turns on, this concerns me cause I'm just a student and I cant afford to buy a new one, my phone had 0 issues, till suddenly it keeps on turning off, it's not a battery problem either, please fix this apple, i have important sms messages i need to receive, but again i can't see it cause my phone is dead

[Re-Titled by Moderator]