First - don't panic! The best thing to do is to first take your missed pill as soon as possible. You can continue to take your remaining pills at the usual time, even if this means you take two pills in the same day. Emergency contraception is usually not needed.
If it has been multiple days, you should take the most recent missed pill as soon as possible and throw away the other missed pills. Then, continue taking your remaining pills at the usual time. You should use condoms (or a back-up method) during sex or avoid sex until you have taken the pills for at 7 continuous days.
Fore more information, visit: https://www.cdc.gov/reproductivehealth/unintendedpregnancy/pdf/248124_fig_2_3_4_final_tag508.pdf.