Google "I lost my stimulus payment and need a replacement"...or something like that. I did it a while ago and found a phone number for a client. Might warn them in advance that they have to be willing to give out their SS# over the phone.

Or fax in Form 3911.   

If you google 'AARP missing EIP' they explain how to fill out the 3911 for the missing stimulus payment

PS: the letter has instructions on it.