If you have a lost receipt, your best plan of action probably is to contact the place that gave you the original copy and see whether the store can reprint the receipt for you. When getting a reprinted receipt isn't an option, you might be able to check back through your bank or credit card statements to see whether you can find a copy of your transaction, but of course, this is helpful only if you paid for something with a check, debit card or credit card. If you need to return an item and are unable to prove your purchase, you still might be able to get a store credit. Most stores will not give you cash on returned items when you don't have your receipt, but a store credit will allow you to purchase another item of equal or lesser value.
Very few companies give receipts to customers without keeping copies for themselves, so losing one isn't necessarily an irreparable problem. If you can't find your receipt, visit the place where you made your purchase and request a reprint. Many places of business are hesitant to do this for people because it tends to be a lot of trouble on their end, particularly for big companies that have lots of daily transactions that would need to be sorted through. Try to remember all of the details that you can about your purchase before you ask for a reprint. Knowing the date and the amount of your purchase as well as a description of the person who took your money might make finding your specific transaction much easier for the company with which you are dealing.
If you used a debit card, check or credit card when you made your purchase, you probably don't have to have your original receipt. Check back through your credit card and bank statements to see whether your purchase shows up there, and if it does, you can print copies of your statements and use those as proof of purchase. Most companies will accept this as a replacement for a lost receipt. If you are unable to get your money back when you return something, you might be able to get a credit from the store in question. This might not be offered to you immediately, so you should ask for a credit if it isn't offered.
There are a few things that you can do to prevent losing receipts in the future. If you have a cell phone with a built-in camera, you can take a picture of your receipt as soon as you receive it. It's also smart to scan your receipts into your computer if you own a scanner. Most companies will accept images of receipts if the physical copy is not available. Be especially careful with receipts for expensive items, such as jewelry, electronics and tools, because these things are much more likely to cause you lots of stress if something happens and you need to make a return.