Singing is one of those things that mood can control to quite a large degree, so often it's your motivation that's impaired not your vocal ability.

The time of day can also be critical - most people find that their voice tends to be better later on, athough I knew one soprano who could hit top D's in the morning, but not later.

Of course there is the fact that your hearing might be the variable.
There is an old joke about the man who went to his doctor and said "Docter I suffer from terrible flatulence, but happily my farts are silent and odourless". The doctor prescribed some tablets and told hime to return the following week. The following week the man told him, "Those tablets didn't help at all I'm still continuously farting, but now they smell terrible". "Well", said the doctor, "That's sorted out your sinusitis, now we can work on your deafness.