Make Up for Forgetting Someone's Birthday

Our current time is full of so much stress and work, it's no surprise that someone might forget a friend's birthday.

Relax. Once you remember their birthday, don't panic. Take deep breaths.Call some friends. They should help you and give you advice. Make sure you call people who know the person of who's birthday you missed.
Schedule to hangout with the friend. If they are staying at home, try to get them out of the house. Some good ideas are seeing a movie, going shopping, or just hanging out.

Don't bake a cake. If you are a fairly good cook, you can try baking one. If you have never touched a spatula in your life, don't try. Instead, order one from the nearest bakery.
Buy things that the friend likes. You can get them their favorite candy, an extra gift, or anything you think they might enjoy.
Give them what you've bought. Apologize for missing their birthday, and hand them the gifts. Hopefully they'll accept your apology, and you'll be good to go!

If you remember it and have no time to buy gifts, simply apologize to the person and try to hang out with them to make up for it.
If your friend doesn't accept your apology, don't get angry. You did miss their birthday, which is an important time for anyone. Let them sleep on it and apologize later. However, don't pester them.