I would call customer service, and calmly (without the ALL caps and exclamation point excitement that your post implies) explain what happened, and that you would like to restore your account to be able to pay online. If the rep is not cooperative, thank them, hang up and call back. Eventually you will get someone who can help.
You may have to go through the explanation a few times, but I think if you present yourself as reasonable, and can calmly explain what happened, you'll get your account status righted.
You need to speak with financial service department. Customer Service Rep, does not have option to change you back to no cash only status. All CSR can do is speak with them on your behalf, to see if they can convince to change you back due to the issue at hand.
Cash Only status is given when you have three or more returned payments in the last 6 months, when you have any payment returned within the first 60 days of account activation OR on accounts with a return payment of an amount greater than $500. Do any of these apply to you? If so, in these situations VZW reserves the right to put your account on a Cash Only status. Only a CFS (Financial Services) representative has the ability to remove Cash Only status from your account so you would want to speak with them if you have a dispute.