Review these checks and tests in about the order given to find out why your light is not working. I'm listing steps in an order guided by a guess at probability of the problem cause along with ease or simplicity of each step.

1. The bulb: take a bulb that's known to be working - you saw it lit on a fixture - and try it in the fixture - if it won't light then try it again in the fixture where it was working to confirm the bulb is still good, OR try the existing light's bulb in an indoor fixture - we want to avoid looking dumb because we had a bad bulb
2. Switches: check that all relevant switches are on
3. Electrical power: check for electrical power right at the fixture, using a DMM, VOM, or neon tester.
   
   Watch out: if you don't know how to do this ask for help from a licensed electrician as you could be killed.
4. Corroded electrical lamp socket base or sides: if your outdoor light or floodlight is flickering or goes out entirely the cause may be the obvious ( a burnt-out bulb) but it might be more subtle: corrosion in the bulb socket.
   
   Test the floodlamp bub in another fixture: if the bulb lights then the problem is in the bulb socket or the fixture itself.
   
   With electrical power to the light switched OFF and confirmed as OFF using a neon tester or other test device, try cleaning, scraping, sanding the bulb contact points and then using a small screwdriver, gently lift the center contact point a few millimeters up to assure that it makes good contact with the bulb base.
   
   Watch out: if you don't turn power off before trying to clean up or replace a bulb fixture you could be shocked or killed.