One of my saved invoices now won't let me add to it?

Hello chanellmandymagp, thanks for joining the Community

Do you receive any specific error message when attempting to make or save changes to the invoice? 

To rule out any browser issues that may be impacting this, please try logging into QuickBooks via a different web browser or incognito browsing window. 

It's also worth checking the close your books setting if your user has access to this. When turned on, this locks the books so no one can edit the accounting data before the closing date.