Sometimes students say they are "too sick to sing" because they have a cold, and they let their practice slide. Even when you are struck by the common cold, try 2 things:
- mental practice. Imagine yourself singing the pieces, see how many mistakes you make. Do it in real time. This helps, believe it or not!
- slow, low, gentle warmups like humming, oo, ee. Use only the easiest part of your range and concentrate on getting clarity in your tone.