I have a burning pain above the left leg above knee since last 5 years. it has also loose sensation. what is the cause

Burning pain is usually due to nerve injury.  Leg pain can originate from a back injury irritating the spinal cord.  Burning pain can be leftover from shingles or reactivated chicken pox.  This is known as post-herpetic neuralgia.  Some vitamin deficiencies can also cause burning pain.  Long-term uncontrolled diabetes can damage nerves and lead to burning pain but typically in the feet rather than above the knee.  

Just because you have no symptoms of diabetes, liver, spleen or kidney problems doesn't mean that you don't have diabetes, liver, spleen or kidney problems.  Often times, diabetes, liver & kidney problems don't manifest with symptoms until it's too late and you have full fledge disease that is more difficult to treat than at an earlier stage.  That's why it's important to discuss your concerns with your family physician and get appropriately evaluated.  Good luck!