All clients that relied on the testnet Hiro API. This includes the Explorer, Stacks desktop and web wallets, the stacks CLI, and others. Requests on all endpoints to the testnet API were still being served, but we showing the same block and transactions for some time.
This was determined to be caused by a Testnet Bitcoin block storm. See a writeup on what these are here:
After the storm subsided, testnet mining resumed and blocks started progressing as normal.
There are a couple issues open on the stacks-blockchain repository aimed at reducing the impact of Testnet Bitcoin block storms on the Stacks testnet network