It's disappointing to retrieve your sweatpants, warm and snuggly from the dryer, only to find the end of the drawstring has disappeared into the channel of the waistband, or it fell out ... somewhere at the laundromat. Drawstring fixes are easy and the tools are easy to come by.
Drawstring Lost in the Channel
If one end of the drawstring is reachable, pull the drawstring all the way out. After ensuring there's a knot at one end to prevent it from disappearing again, restring it.
If both ends are lost in the channel, feel for the nearest end, and pull it out using a household remedy: Bend a small hook at the end of a piece of very straight wire. If you use coat hanger wire, be sure to straighten it well enough that it won't get stuck in the drawstring channel. Or if the string's end isn't too far away, you may be able to snag it with a crochet hook. Pull it out. Then restring the pants.
Restringing Your Sweatpants
Use a large safety pin -- probably the oldest method -- or take a tip from the Meredith Vieira Show and use a pencil to tunnel the drawstring through the waistband.
Safety Pin Method
Pin one end of the drawstring with a safety pin -- find the largest one that will easily slide through the channel. Push the safety pin, drawstring in tow, into the waistband channel.
Grasping the pin from the outside of the pants, and pinching some of the material with the other hand, move the material up the pin. Then pull the pin forward within the channel, and repeat this process until it's made it through the other end.
Securely tape about an inch of the threading end of the drawstring to a pencil. Push the pencil into the channel.
Working with your fingers on the outside of the waistband/drawstring channel, bunch the material onto the pencil, pushing toward the eraser end, and pulling the pencil further into the channel, as you would the safety pin.
This fix is simple -- and if you have a long enough shoelace on hand, it's a quick fix too.
Repeat the process with the safety pin to work the new lace through the waistband. Remove the pin, and clip off the plastic ends of the shoestring. Then tie knots in each end, large enough that they cannot slip into the waistband again. Or sew a button on each of the ends, as recommended in, Handy Household Hints From Heloise.