my iphone was restarted and it came back sim locked how do l unlock it

You contact the carrier that the phone is locked to. That is the ONLY way to unlock an iPhone. If the phone was not SIM locked when you bought it it must have been hacked by the seller to bypass carrier lock. You can try asking them to repeat what they did, but note that it means your phone is hacked, and its security and safety have been compromised.

See→How to unlock your iPhone for use with a different carrier - Apple Support

If you forgot or don't know your SIM PIN

Never try to guess a SIM PIN or PUK code.* The wrong guess can permanently lock your SIM card, and then you would need a new SIM card. If your device asks for a PIN or code that you don't know, here's what to do:

1. Contact the carrier that gave you the SIM card. If you aren't sure which carrier to call, remove the SIM card and check the card for the carrier's name or logo.
2. 
3. Ask your carrier to help you unlock your SIM card using the default SIM PIN or PUK code. 
4. If you can't unlock your SIM card using the SIM PIN or PUK code or if an alert says, "PUK exhausted," ask for a new SIM card.

*If you enter the wrong SIM PIN too many times, your wireless carrier might be able to give you a Personal Unlocking Key (PUK).