All clients that relied on the mainnet Hiro API. This includes the Explorer, Stacks desktop and web wallets, the stacks CLI, and others. Requests on all endpoints to the mainnet API were unable to be served.
An unoptimized and deprecated endpoint on the API saw a spike in traffic over Sept 20th, causing our APIs to consume all available memory and crash, resulting in one very short moment of unavailability (~5 seconds).