1. Remind her of the good times. Now, this option is very person-specific. A lot of women catalog things in their minds based on how they feel. Take a moment to let that set in. Yes, they base things on how they feel, which means you need to do something that'll remind her about a great time you two had together. It could be visiting the park where you first told her you loved her. It could be reproducing a great date you had once. It could be something as simple as picking up her favorite movie and ordering pizza, if it has sentimental value to her. Think hard, you'll come up with something.
2. Problem specific. Be honest guys, do you take your sweetheart for granted sometimes? Does she cook for you? Does she clean? Has she spoiled you to the point to where you expect things from her? If this is the case, you grown men need a self intervention. Share in the cooking duties, and totally take over the cleaning responsibilities. Now, you can't just handle said duties once or twice. Incorporate them into your daily routine. It'll take some time. but eventually she'll notice and love you once more.
3. The Kobe route. Remember when Kobe Bryant got caught cheating on his wife, for the first time? He spent boatloads of cash on his wife just to keep her. Well, you may not have Kobe Bryant money, but you can follow the Kobe Bryant rule. The bigger you mess up, the bigger the gift. Take her on a cruise to a place she's been begging to go. Spoil her. Treat her like a queen. These are things you probably should've been doing in the first place. The gift needs to be as specific as possible. Don't by her furs if she's down with PETA, you know?
4. Something new. Is there something she loves, and you completely despise? Is she into opera, and you'd rather be watching the Bulls play the Knicks? Show her you care, and immediately get back into her good graces by blowing off something you love to do. Putting aside your videogames, just once, to indulge her desire to attend some event you have absolutely no interest in. Don't complain once about missing anything. Fake like you're having a good time. You know she will be. By the end of said event she'll have a smile on her face, and she will have reopened her heart to you. And next time, try not to muck things up so badly.