Apologize first

I'm very sorry, but I won't be able to come in to the office today.
I'm really sorry for the short notice, but I have a problem.
I hate to put you in a difficult situation, but I won't be able to get in today.

Have an acceptable reason

… My car's blocked in.
… My car won't start and I'll probably be a bit late.
… My son / daughter has a temperature and I have to stay at home.
… I'm not feeling very well at all.

Make sure your message gets to those who need to know

I have a meeting at 10 with X. Can you call them and let them know?
Do you think you could tell X and Y for me?
Would you pass on my apologies to X? We're supposed to have a meeting this morning.

Say when you'll return to the office

I should be back in tomorrow.
I hope to be in this afternoon.
I'll let you know as soon as I've seen the doctor.