Q: My Sim Card says it is not installed what do I do, and how do I fix it. Please let me know ASAP!

try opening the sim card tray and wiggle the card about a bit and close it again

to see if that give a connection

then try another sim card to verify if the sim card in it is defect or if the phone is sim locked to simcards from another operator

then contact apple rapair if some of the connectors which connect to the sim card have been damaged the phone would require repair