Q: I changed my SIM card in my phone and it says no service.What do I do?  I have rebooted it 3-4 times.

Is the Sim from the same carrier as before?

Is your iPhone carrier locked?

If so, changing the SIM will not unlock iPhone.

Usually, if the carrier sends it to you (as opposed to getting a new sim at their store/kiosk, etc.), the old sim needs to be deactivated and the new one needs to be activated (carriers usually do not send active sims through the mail/courier), particularly when your phone number remains the same (two sims with the same number cannot be both active at the same time).

Call your carrier and have them deactivate the old sim and activate the new one (you may have to read them out the tiny serial numbers but usually those numbers are also on the larger punchout card.

Some carriers allow you to activate sims online (usually those instructions would be with a booklet accompanying new sim though).