Switched SIM cards but contacts are no longer on my iPhone?
SIM cards can store a certain amount of contacts, but most phones nowadays do not automatically store them there, instead storing them on the phone itself. 

If your old phone has stored your contacts on the SIM card, you can import them onto your iPhone in the Settings app: 

Go into Mail Contacts and Calendars, scroll down to the Contacts section, and tap on Import SIM Contacts. 

Note: iPhones CANNOT store contacts onto the SIM card. If your previous phone was an iPhone, you will need to sync them by some other method. 

More Info: Apple Support Doc