Dogs get lost. It happens. Sometimes it is deliberate (as in someone lets your dog out of it’s yard or unties it). Sometimes it is accidental – car accident, gate left open that shouldn’t be, etc. Sometimes your dog is just one that likes to escape.
It is good to know what to do should such an event happen.
Searching for your lost dog is a good guide to be aware of. It’s a PDF file so you would need to download adobe acrobat.
Dog Detective has posters one can use to assist in the search.
FidoFinder also lists some good ideas to keep in mind when missing one’s companion. They start from the basics of first having to make sure your dog is actually gone missing. Sometimes dogs can just get so intrigued by something that they don’t come when they are called, and if you have sufficiently large property it may take you a while to locate them. Only after you have determined that your dog is gone should you act, but act quickly! Time is of the essence.