hi welcome to HCM.do you have fever,swelling of knee or any symptoms along with knee pain? sudden knee pain with out any associated diseases may be due to tendanitis,sprain
and many causes are there.Take 800 mg of ibuprofen
or Advil) every 6-8 hours if you are not allergic to it. This should help the pain and swelling.If you don't notice any improvement in 2days or if pain keeps on increases, see your doctor or a specialist in orthopedics.