There is a little tray with a hole in it on the right side of your iPhone 6. After turning your iPhone fully off (press and hold power/wake button on upper right side until Slide to Power Off slide comes up. Turn off with the slide. Push an unfolded paper clip into that hole and the tray will pop out. With a microfiber cloth rub the contacts of the SIM card. Reinsert the card. Turn phone back on by pressing and holding the power/wake button again until Apple logo appears.
If that doesn't work, you will need a new SIM card from Verizon. If you go to a corporate store, these are free. Just bring your phone in with you and explain the problem and what you did so far.