I am not receiving emails. Why is my account suddenly saying there isn’t enough storage after nearly 2 decades of use, and when I’ve deleted 100’s emails?

Question

Within the last week, completely out of the blue, my account is no longer receiving emails. The account is saying that there isn’t enough storage, despite me deleting 100’s of emails. I’ve had the account nearly 2 decades and the amount of mail I have has never been an issue. (The amount of junk mail I have to delete daily has been an issue for me, but I’ve been keeping up with deleting that and that’s another story!)