I think my belly button ring has rejected it’s completely fallen out and the piercing looks swollen and red do I try to put the piercing back in or let it close and get it redone?

it closes as it slowly rejects so there’s really no saving it without hurting yourself more. let your body heal and eventually try to get it redone if you want it

it may be infected so i would go to a doctor to check and if it is infected you’ll need to get antibiotics

See your piercer as well to see what the best course of action is