A text left on read is not an emergency. Most people are just busy, not plotting.

Silence is the easiest thing in the world to fill with the wrong story. Phones make it worse by showing exactly when somebody saw your message, so an ordinary busy afternoon becomes evidence that something has gone wrong. The usual explanation is the boring one: they meant to reply and then did something else. Your worry is not information.

Give it a day before deciding anything, and keep your hands busy meanwhile. If it still matters tomorrow, ask them plainly rather than reading the timestamp again.