Make sure you don't have airplane mode turned on. Make sure you have a SIM card in the phone. (I've seen "friends" steal SIM cards to save money.) If everything looks good, turn the phone off, remove the SIM card (notice which way the cut corner goes - if you're not sure you'll remember, make a little sketch of it - you just have to know which corner the notch is at, and whether the card is facing up or down.) Wipe the contacts with a clean cloth, from the card out. Just a few wipes, so you wipe all the contacts. Put it back in and try it. If it still doesn't work, bring the phone to your carrier and ask for help. (The SIM could be bad, your account could have gotten messed up or the phone could have failed. If it's one of the first two, the carrier has to fix it - either replace the SIM or reset your account.)