I have send some messages to recipient while she had deactivated her WhatsApp account but now she's back still those single-ticked haven't been delivered. I just need help to deliver those messages because I want her to know I have been messaging all these days.
If she deactivated before 30 days the messages won't get delivered, because WhatsApp stores the message in the server for 30days. Read it here, Even I haven't tested -> Scroll to update section until it gets delivered.
If she activated within 30days probably with no DP or status she should have blocked you.