Is there somthing dead in the tank? Sounds like you might have a dead fish or some inverts somewere. That is slowly decaying and causeing your nitrates to rise.
Tanks been going good for about 2 years. Have tested twice and changed a couple of things that may have caused a spike waited a few hours and retested still no change.
Assume you checked your water supply; didn't stir deep sand bed; checked fuge to make sure it hadn't become a nitrate sink, etc. Given how suddenly and dramatically your values changed it might be worth getting another brand of test kit to check the validity of your results.