The reasons could be varied, however, your boss wants to hear a legitimate excuse. If you are honest, you run the risk of being labeled unprofessional. That's why you have to come up with some seemingly real and genuine reasons for not being on time.
I Am Not Feeling Well
Nobody can beat that. If you are a woman, you have a solid reason like "pain in stomach". If you have a male boss, he won't even want to go into the details. Otherwise, "having a cold" is one of the best gender-neutral excuses. Call your boss first thing in the morning after waking up, your voice will be hoarse so it will really seem like you have a cold.
My Bank Called
No office would want their employees to be in financial trouble. Call up your boss and tell him that you had applied for a personal loan and the bank manager called up to meet up urgently as there were some issues with your documents. As you really needed the loan, you had no choice but to go to the bank and sort out the issue.
My Dog Was Lost
Tell your boss or teacher that your pup ran away and you were not able to locate him until the morning. Tell them that you were frantically searching for him around your house, and on the streets. You were unable to sleep, and were consoling your sister who was crying uncontrollably. Only a hardhearted teacher or boss would not be convinced by this.
Could Not Hear the Alarm
Tell your boss that you were watching a late night show since you worked for ten hours in the office the day before. You were so tired that you fell asleep on the sofa. The alarm clock was in the bedroom, so when the alarm rang in the morning you could not hear it. You overslept, and hence will be late for work.
There Was a Traffic Jam
Use this excuse only if you take a route to work which is jammed during peak office hours. Otherwise, tell your boss that you were staying at a friend's place for the night who stays on "X" road (name a populated, suburban place). You miscalculated how much time it would take you to get from his place to the office and hence you are reporting late.
I Misplaced My Car Keys
To make it sound genuine, tell your manager the story right from how you began your search, checked the shelves, cupboards, tables, driveway, and then finally found your keys. The more details you give him, the more he is likely to be convinced.
My Car Broke Down
This can happen to anybody. After all, machines can be unpredictable. Stand on the balcony of your house or any place where traffic sounds can be heard. Call up your office and say that you are stuck in the middle of the road as your car refuses to budge. Let your manager hear the traffic sounds in the background to make all this seem genuine.
My Daughter Fell Ill
If you have children, this is the perfect excuse. If not, tell that your sibling, spouse, or mother fell ill and you had to take her to a doctor in the morning. After that, you made her soup, waited while she ate, and then put her to bed before you reported for work. Your boss won't be too upset with a family man.
My Neighbor Got Locked Out
Tell your boss that your next door neighbor left her keys inside her house and accidentally locked the door from outside. Her baby was crying inside the house and the mother got really scared for him. You saved them by getting the door unlocked.
There are a number of excuses that you can come up with, you just have to think out of the box. Blame it on the neighbors, on your alarm clock, or on your failing health and enjoy a hard-earned extra hour of sleep once in a while.