The running thread will be preempted when a higher-priority thread is placed on the ready queue (it becomes READY as the result of its block condition being resolved). The preempted thread is moved to the start of the ready queue for that priority, and the higher-priority thread runs. When it's time for a thread at that priority level to run again, that thread resumes execution — a preempted thread will not lose its place in the queue for its priority level.