Do you mean that you did have a tracking number, but lost it? The receipt will have that number on it. Look it up yourself on USPS.com to see how far it got, before it was lost. Then take the receipt to the post office so they can take it from there.
If you didn't get a tracking number, you could ask them to try to trace it, but it's a long shot. At least you'd have the date it was mailed, which would give them a little info to go on.
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