Before asking why your camera won't turn on, or your lens won't focus, try the obvious things:
- are batteries charged?
- take the batteries out, check for corrosion, reinsert them
- if your camera has a system reset (most do) try resetting to factory settings
- if you have interchangeable lenses, try removing the lens, cleaning the contacts, and firmly re-seating it
- if you're not sure if the problem is the camera or the lens/flash, try another lens/flash to narrow down if it's the camera body or
When asking questions, make sure you mention everything you've already tried.