It seems that you are suffering from a depression. Depression is a desease affecting millions of people which can be successfully surmonted.
You must contact your physician who after an exam can prescribe an antipressant medication to you. Within two to three weeks of taking the medication you should gain your good humour back. Sometimes, a follow-up with a psycho therapist is of great help so that you can quit the medication.
You should not delay in consulting your doctor as depression may get worst with time and may lead to despair.