Why is my pay as you go phone saying no sim card?
A no sim card message means that the phone is not physically able to read the sim card. Removing and reinserting the sim card, as Sophia suggested, may restore the connection, (although you will likely have to restart the phone as well). If it does not, there may be an issue either with the phone or the sim. You can test which by either putting this sim in another phone, or trying another sim card in this phone.