Well, I have to make some assumptions before I can say what I would have done. I assume that you mean the libero was in for (had replaced) the middle after he/she lost the serve. If that is the case, and the front-row middle was to rotate into the back row, then you have several violations.
1. Illegal libero replacement. If the libero was in for the middle (who came onto the court) then the only way that the libero could stay on the court would be to serve for the middle coming off (assuming that the libero had not served in any other rotation). If the libero does not serve and stays on the court, it is illegal because the libero must sit out one rally before replacing another player.
2. Illegal substitution. Assuming that the middle was to rotate into the serving position (and everything in #1 was correct), the serving team needed to substitute in order for someone (from the bench) other than the middle or libero to serve.
3. Incorrect server. Assuming that the middle was supposed to rotate into the back row to serve AND someone already on the court (other than the libero) actually served, then you have an incorrect server.
Again, I'm assuming that the middle playing front row was supposed to be the one to rotate into the back row. If a different player was supposed to rotate into the back row, then you can add in illegal alignment as another option.
I suppose that you could whistle the play dead after the R1 beckoned for serve (for illegal libero replacement) but I'd wait until the serve was contacted before killing the play. Then I would take my pick between 1, 2, and 3
. After that, I would expect to have to take a couple of minutes to explain what was wrong and get everyone into their correct positions.