Weakness is a symptom used to describe a number of different conditions, including: lack of muscle strength, malaise, dizziness or fatigue. The causes are many and can be divided into conditions that have true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.

Causes of Weakness
Many different conditions can cause weakness and fatigue. In 2010 DiagnosisPro listed 464 possible cause that can cause fatigue. True weakness may be due to problems with the nerves, neuromuscular junction or with muscles.
Thrombocytosis
Amyotrophic lateral sclerosis
Botulism
Centronuclear myopathy
Myotubular myopathy
Muscle Atrophy
Sarcopenia
Dysautonomia
Charcot-Marie-Tooth
Hypokalemia
Motor neurone disease
Muscular dystrophy
Myotonic dystrophy
Myasthenia Gravis
etc....

Home Remedies For Weakness And Fatigue
Poor sleep may cause weariness, so also, a stressful way of living.
Streamline your diet. Avoid eating too much of junk food and outside food. Eat a balanced diet consisting of all nutrients.
Lack of exercise can cause fatigue. Doing regular exercise will increase oxygen storage capacity of the muscles and also strengthen the immune system.
Practice relaxation exercises such as yoga, meditation, and deep breathing.
Sometimes fatigue can develop as a result of mental stress. Face up to work and relation problems and do something to resolve them. Talk to a counselor if necessary.
Above all you have to prevent dehydration. Drink at least 8 glasses of water in a day.
Eat frequently and small meals. Eat at least small 5 meals in a day at an interval of 2 to 3 hours.