What will happend if I will miss my pills for one day?

Which pill are you on? Please add answer as a comment and I'll try to help.

Generally though if you are taking a combined oral contraceptive:

If you miss one "active" pill, take two pills on the day that you remember. Then take one pill per day for the rest of the pack.

If you miss two "active" pills in a row in week one or two, take two pills per day for two days in a row. Then take one pill per day for the rest of the pack. Use back-up birth control for at least 7 days following the missed pills.

If you miss two "active" pills in a row in week three, or if you miss three pills in a row during any of the first 3 weeks, throw out the rest of the pack and start a new one the same day if you are a Day 1 starter. If you are a Sunday starter, keep taking a pill every day until Sunday. On Sunday, throw out the rest of the pack and start a new one that day.

If you miss three "active" pills in a row during any of the first 3 weeks, throw out the rest of the pack and start a new pack on the same day if you are a Day 1 starter. If you are a Sunday starter, keep taking a pill every day until Sunday. On Sunday, throw out the rest of the pack and start a new one that day.
If you miss two or more pills, you may not have a period during the month. If you miss a period for two months in a row, call your doctor because you might be pregnant.

If you miss any reminder pills, throw them away and keep taking one pill per day until the pack is empty. You do not need back-up birth control if you miss a reminder pill.

yes, the possibility of her getting pregnant is much higher now that she has missed a dose. It will take 4 weeks until her body is fully protected again.

If forgetting pills happens often, look into other forms of birth control such as the depo provera shot, IUDs or the new Nuva Ring. These are all as effective against birth control as the pill except the nice thing is you don't have to remember to take them at the same time everyday!