Suggest Treatment For Difficulty In Straightening Knee

For the past week or so i have been experiencing some problems with my left knee. This began at night when i was sleeping, i was unable to turn over due to being unable to straighten my knee without severe discomfort. 1 week on it has got increasingly worse and when straightening my leg there is often a clicking/grinding or popping sensation which is agony. This tends to be mainly at night however i have experirnced it more frequently at work also, please help!