Missing placebo pills doesn't matter. The placebo part means they're completely inactive. As for missing a single pill, read your package insert (you can find it online if you threw it out) to see what it says about missed pills and whether you need backup.
Even pills taken perfectly can fail, so if it's a week past when you should have gotten a period, you might want to take a test just in case, although most pills say take a test if you have two missed periods in a row.
There's always a small risk of getting pregnant having unprotected sex while on birth control, and even more of a risk if your partner ejaculates inside of you. Putting this small risk aside to answer your question -- you will not get pregnant during the placebo pills if you have been taking your birth control continuously the entire month. There is a chance of pregnancy if you miss any of the white pills. You should take the one you missed with your second pill as soon as possible. If you have had sex within the last couple of days, you can buy the morning after pill and take is asap.
Take a look at the information listed on the Planned Parenthood website about missing your birth control pills:
Here's the risk factor statistics from Planned Parenthood for different birth control methods:
Have you ever been late for your period before? Most women get their period during the placebo week. Are you taking the 21 or 28 day pack?
I usually start the exact same day, on a Wednesday, during the placebo week but I have had my period start a day before or a day late. When I was a teenager, my period was always on a Sunday. I've been on birth control for 18 years and it gradually changed days.
I suggest take the pill asap, go get the morning after pill if you had sex within the last few days. Buy an early detection pregnancy test.