What do I do if I forgot my Stellar memo?

if you are making an ‘everyday’ transaction on the Stellar network using a popular Stellar wallet like Infinito 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. 

More information here