If you are making an ‘everyday’ transaction on the Stellar network using a popular Stellar wallet, forgetting the memo is no big deal; it’s optional. If you are sending (depositing) lumens to an exchange, forgetting the memo will generally lead to your exchange account not being credited with lumens. If this happens, it’s important that you contact the exchange you sent lumens to — SDF can not help you recover your lumens.
We suggest finding your transaction using the Payment Locator on Stellar.Expert and getting as many details as you can regarding the transaction. Reach out to the exchange’s support team with the transaction details, let them know that you forgot the memo, and see what they can do to help you identify your transaction.
Going forward, make sure to review the address you’re sending to, verify if you need to provide a memo, and check that you provide an appropriate fee. You should only send transactions after you have double checked all of this information and verified that it is correct. Remember, it’s never a bad idea to try sending a small amount of an asset before making any transaction.
Read more here.