Since you could have missed a pill a good number of days before you had sex, then yes, you could be at risk. Most pill-pamphlets say that if you miss a pill you are still protected, but only as long as you remember to take it the next day.

Having that missed pill could have caused your hormone levels to drop enough to ovulate, since that day without hormones could have caused a sort of "blip" in your hormone levels.

You probably should have waited 7 days to have sex, so the levels could build up again and to ensure you did not ovulate, BUT, since you only missed one pill, there is not as much of a concern as if you missed more pills.
Technically speaking, this definitely puts you in the "typical" use rate for the pill (which refers to people who sometimes miss pills, take their pill very late, etc), which puts you at about 92%-97% protected.
Also remember that actually getting pregnant is a very complicated process that requires everything to go exactly right, and your pill's job is to make just one thing go wrong to prevent this (whether it be suppression of ovulation or thickening of cervical mucus so the rare case of a fertilized egg would not survive).
Best of luck, hope this helped!

Also, you can take a pregnancy test 17-18 days after sex and get an accurate result.

One more thing-- for future use, emergency contraception is a great form of after-the-mistake protection. You can get it at a pharmacy, under the name Plan B, but you can also use your regular birth control pills for EC.

if you only missed ONE pill you should still be ok...however, you should have taken that pill as soon as you remembered...honestly..i think you're fine though...