The way how invoices were sent to customers affect the availability of the invoice or payment link. Usually, when you enable your account for e-invoicing, the Pay Now button is added to the invoice or reminder sent to the customer. If the link becomes unavailable, the email preference may be set to HTML or Plain Text.
Let's try checking your Account and Settings and seeif there are changes made to your Sales Preferences. I'm here to help guide you on how.
Open your QuickBooks Online account.
Select Account and Settings from the Gear icon.
Choose Sales on the left.
Click the Pencil icon in the Online delivery section.
Check if the Online invoice is selected under Additional email options for invoices.
Tap Save, then Done.
Once completed, try sending an invoice reminder from the Invoices tab.
Go to the Sales menu at the left pane, then select Invoices.
Click into Overdue Invoices, then select the box field next to the invoices that need to be sent to customers.
Choose the Send reminder button or Share invoice link under the Action column.
Besides, I would recommend checking that the payment options appear on the invoice and have a check-mark placed in the box next to them.