While there have occasionally been system infrastructure outages, most of the outages over the past few years have been in banks' systems. Sometimes it has been their account systems affecting the ability of customers to make payments. Sometimes it has been their merchant-facing systems so that merchants were unable to take payments.
In these cases, often the only fall-back at the point of sale is cash. Prior to COVID-19, there was already a sizeable proportion of people who tended to have little or no cash in their wallets. So when disruptions did happen there were stories of people leaving goods at the counter and merchants effectively having to close until the systems were restored.