I had a bad toothache for one day then it went away. what could this be?

A problem: There is no way for us to know if you have a problem in that tooth, and whether it's a small or big problem (or nothing). Unfortunately the absence of pain does not necessarily mean there is no problem. See a dentist who can examine you and take an x-ray to determine what, if anything is the problem. We can only guess while your dentist can tell you for sure.

See a dentist: You may have a broken filling a cracked tooth an abscess and many other things. I wish it was that simple to know why your tooth was hurting but it's not. To have a definitive answer you need to see a dentist and have a complete examination done.

A Warning Sign: A bad toothache tells you that something is wrong and that you should see a dentist ASAP to diagnose and treat the problem before it gets worse. Left untreated, you could be setting yourself up for more pain and more significant problems.