Unless someone has executed a CPU HLT (which is very, very rare) your CPU is actually always running at 100%.
When the O/S reports a CPU usage percentage, it is reporting a portion of the CPU cycles that are used for a certain type of thread (user, kernel, or both). There is another type of thread (the Idle Thread) which is always ready to be scheduled-- idle threads are not included in the CPU calculation. When the O/S says 75% CPU utilization, that means the idle thread was running for 25% of the clock cycles.
So if your question is-- can a CPU actually run at 0%, literally-- that is very rare unless you are issuing low-level hardware CPU instructions.
If your question is-- will the O/S ever report 0% utilization for user threads-- yes, this is certainly possible if you kill all your processes (and all those processes belonging to other users, including service accounts).
If you question is-- will the O/S ever report 0% utilization for both user and kernal threads-- yes, it is possible, in theory, if you have turned everything off and also killed all the processes that allow the O/S to function (e.g. receive keyboard input). You wouldn't be able to do anything with a system in that state. Meanwhile, the idle thread of course will still be running.