Picture this scenario. You've been burning the midnight oil on a project at work but suddenly realize you've totally forgotten about another project. A quick glance at your calendar puts a lump in your throat -- the project is past due. Missing a deadline isn't the end of the world, but it can send a negative message to your immediate supervisor. You're not, however, the first person in your office to miss a deadline, and provided you deal with it through a proper apology and careful attention to detail in the future, it shouldn't derail your career.
Speak to your manager immediately after you realize that you've missed a deadline. Although it's also important to follow up with a letter, you don't want to spend time on the letter while your boss is wondering why you haven't addressed the situation. Explain your reason for missing the deadline, respectfully apologize for any inconvenience it might have caused and stress that you'll get the project finished as quickly as possible. Agree on a new deadline -- and ensure that you meet it.
Write an email to your manager to formally apologize for your mistake. Take responsibility for the situation with sincere wording, noting your reason for missing the deadline and stressing that despite your actions, you value deadlines. Explain how you'll learn from the issue and not let it happen again. For example, you may write, "This unfortunate issue has led me to be vigilant about updating and monitoring my personal calendar so this won't ever happen again."
Meet all work deadlines going forward to clearly show your boss that you were serious about your pledge to be vigilant. When possible, complete projects earlier than the deadline to demonstrate your attention to detail and awareness of schedules.
Use an online calendar to set periodic reminders that help you meet deadlines. For each long-term project, set your calendar to alert you one week before the deadline to help you wrap up the work.
Although you might be tempted to show your remorse for the issue by elaborating on the detrimental effect your actions have caused, avoid doing further damage to the situation. Dwelling on the missed deadline can make it seem more significant than it might be.