I tried to send a customer a copy of a paid invoice and now it has disappeared. I cannot find it now, no matter how I try to search for it.

Hi wlagsupply, 

Welcome to Community!  It's definitely concerning when your customer invoices cannot be located in QuickBooks Online.  I'd be glad to assist!

Initially, I suggest clearing cache on your browser to help improve system performance and loading times.  If you're still not able to locate the invoice, go to your Settings ⚙ and select the Audit log, to see if has been edited or deleted.  If it has been deleted, this cannot be undone and you will need to re-create the transaction.  In addition, you can go to Sales > Invoices, then set the Status to All.  

Let me know if this is helpful.  Your success is our number one priority and we are always happy to help!