Life happens. Send in payment now. If they have the late charge on the payment slip...send that in right away - if not, just send in regular payment and expect to get a small bill for the difference. No big deal this one time.
That sucks, but everyone forgets something sometimes. As long as you pay it right away, you should be fine although they may or may not report the lateness on your credit report (some give you 30 days past due, some report immediately, some go longer).
You can probably deal with this without too much trouble as long as this is the first time you've been late or at the very least the first time in a long time. Call them and explain that you forgot and very politely ask for their help. When you ask for help the person to whom you are talking is often motivated to be helpful. Ask what you can do to solve the problem and they may offer to drop any late charge and the report of the late payment. If possible be prepared to pay by phone when you call. I've found this to work on the occasions when this has happened to me, especially if you have a good payment record, good luck.