If you're a smoker, quit smoking. If you're getting over a cold, wait another week or two.

I have the same problem sometimes its good not to smell

Did you snort something you shouldn't have? If so, you may have burnt out your olfactory bulb, and no, there isn't really any sort of a cure. If not, you might have a problem like polyps which can block the bulb and can be removed with surgery.

1) "Anosmia (ænˈɒz.mi.É™) is a lack of functioning olfaction, or in other words, an inability to perceive smells. It can be either temporary or permanent. A related term, hyposmia, refers to decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one particular odor. This is called "specific anosmia" and may be genetically based." "A temporary loss of smell can be caused by a blocked nose or infection. In contrast, a permanent loss of smell may be caused by death of olfactory receptor neurons in the nose, or by brain injury in which there is damage to the olfactory nerve or damage to brain areas that process smell (see olfactory system). The lack of the sense of smell at birth, usually due to genetic factors, is referred to as congenital anosmia. Anosmia may very occasionally be an early sign of degenerative brain diseases such as Parkinson's disease and Alzheimer's disease. Another specific cause of permanent loss could be from damage to olfactory receptor neurons due to use of certain types of nasal spray, i.e. those that cause vasoconstriction of the nasal microcirculation. To avoid such damage and subsequent risk of loss of smell from vasoconstricting nasal sprays, they should be used for only a short amount of time and only when absolutely necessary. Non-vasoconstricting sprays, such as those used to treat allergy related congestion are safe to use for extended periods of time. Anosmia can also be caused by nasal polyps. These polyps are found in people with allergies, histories of sinusitis & family history. Individuals with cystic fibrosis often develop nasal polyps. Causes of anosmia include: Upper respiratory tract infection (e.g., sinusitis or the common cold) Nasal polyps Head trauma, damage to the ethmoid bone Dementia with Lewy bodies Tumors of the frontal lobe Parkinson's Disease Alzheimer's Disease Toxins (especially acrylates, methacrylates and cadmium) Old age Kallmann syndrome Laryngectomy with permanent tracheostomy Esthesioneuroblastoma is an exceedingly rare cancerous tumor that originates in or near the olfactory nerve. Symptoms are anosmia (loss of sense of smell) often accompanied by chronic sinusitis. Intranasal drug use Samter's triad Foster Kennedy syndrome" "Hyposmia is a reduced ability to smell and to detect odors. A related condition is anosmia, in which no odors can be detected. Some of the causes of olfaction problems are allergies, nasal polyps, viral infections and head trauma. It is estimated that up to 4 million people in the United States have hyposmia or the related anosmia. Based on latest scientific evidence, hyposmia might also be a very early sign of Parkinson's Disease." Source and further information: http://en.wikipedia.org/wiki/Hyposmia 2) "Can Smell and Taste Disorders Be Treated? Sometimes a certain medication is the cause of a smell or taste disorder, and improvement occurs when that medicine is stopped or changed. Although certain medications can cause chemosensory problems, others-particularly anti-allergy drugs-seem to improve the senses of taste and smell. Some patents - notably those with serious respiratory infections or seasonal allergies - regain their smell or taste simply by waiting for their illness to run its course. In many cases, nasal obstructions such as polyps can be removed to restore airflow to the receptor area and can correct the loss of smell and taste. Occasionally, chemosenses return to normal just as spontaneously as they disappeared. What Can I Do To Help Myself? If you experience a smell or taste problem, try to identify and record the circumstances surrounding it. When did you first become aware of it? Did you have a "cold" or "flu" then? A head injury? Were you exposed to air pollutants, pollens, danders or dust to which you might be allergic? Is this a recurring problem? Does it come in any special season, like hayfever time? Bring all this information with you when you visit a physician who deals with diseases of the nose and throat. Also be prepared to tell him about your general health and any medications you are taking. Proper diagnosis by a trained professional can provide reassurance that your illness is not imaginary. You may even be surprised by the results. For example, what you may think is a taste problem could actually be a smell problem, because much of what you think you taste you really smell. Diagnosis may also lead to treatment of an underlying cause for the disturbance. Many types of smell and taste disorders are reversible, but if yours is not, it is important to remember that you are not alone: thousands of other patents have faced the same situation." Source and further information: http://www.entassociates.com/smell&taste.htm

As you can see, the answers here vary. The only way you will be sure is to see a specialist.