Sometimes a bird will stop singing. If its feathers are fluffed up and it is sitting quietly then it is probably sick and you need to take it to an avian vet. Sometimes they will stop singing for months if their nails are too long, as this can be very uncomfortable, as well as a health hazard - they can become caught on objects. Trim the nails. Don't cut them too short or they will bleed. For instructions on trimming click here. Maybe the bird is unhappy in a confined space. Get a bigger cage. A healthy, happy canary will sing its little heart out several times a day.
THE ANNUAL MOLT
Canaries stop singing during the annual molt. This happens at the end of the breeding season (around February in the southern hemisphere) for several weeks. At this time they need extra nourishment in the form of protein (feathers are almost 90% protein) and calcium. You can obtain excellent calcium supplements from your vet. Also feed brightly coloured food such as corn and carrot during the molt, as it will affect the colour of the feathers. Vitamin and mineral supplements are also required. At 6 weeks old, young canaries undergo the juvenile molt and replace small feathers. This is normal. For more details on nutrition click here.
An abnormal molt can sometimes occur. Likely triggers are extreme environmental factors such as changes in temperature, shock, illness, fear or stress.