What to do when somebody stops contributing

Ask privately first, and ask rather than accuse. People go quiet for reasons that are usually not laziness, and a private message finds that out without a confrontation. More often than you would think, it resolves there.

If it does not, restate the specific piece and the specific date once, in writing, in the shared channel, so there is a record and no ambiguity about what was expected. Do not silently absorb their work: it feels generous, it teaches the group the deadline is soft, and it leaves you with nothing to point at when marks are discussed.