Run in the opposite direction
Most pups want to be with you. Run after them and they figure, "You're coming too? Great! Let's go exploring!" Run away from them and often, in a few seconds, they're hustling to catch up.
This technique can be made more effective if you clap your hands, whoop it up and generally sound like there's a party that they are missing. When they run up, bend down, praise them enthusiastically, reach under their neck and grasp their collar. Praise some more. No scolding. Scolding after capture simply teaches your pup that bad things happen after capture, and he will try to avoid being caught the next time. You don't fix running away by punishing your pup when he comes to you.
Get interested in the ground
Sounds odd but it can work. Bend down and get fascinated by something on the ground. Poke at it, pick at it, make "Hmm" sounds. Pups are curious creatures and more often than not they will come over to see what they are missing. Avoid lunging at your pup to catch him. This is tempting, but pups are quick, usually quicker than we are. Lunging will make him hesitant to come close next time.
Play with another pup
Every pup has his limits and for many it is seeing "their" person playing with another canine. Throwing a ball for another dog and praising him loudly, giving him treats and generally fussing over him will bring many a pup hurrying back to you.
However you retrieve your pup, count yourself lucky that you got him back. Now that you know more training is necessary, put in some practice. If you have trouble, take a class or find a good instructor. All you need is information; if you are at this web site you already have the interest, love and commitment to make it work. Happy training!