Why won't my Nikon Coolpix L830 camera turn on?

Here's one thing you might try:

Remove the batteries and the memory card. Wait for half an hour or so, and put the batteries back in but not the card. See if the camera will turn on . If it does, switch it off and put the card back in. The camera should now continue to operate normally.

A few points:

• If you've left the batteries in the camera, they may have discharged in the meantime even if the camera was switched off. Use fresh batteries.
• Digital cameras, particularly chunky bridge cameras like the L830, are really fussy about batteries. Only premium quality batteries will work well. Better yet, get a set of premium quality NiMH batteries like the Sanyo/Panasonic Eneloop brand. They last longer on each charge than a set of good alkaline disposable batteries and are much more economical in the long run.
• If the camera refuses to operate normally after you put the card back in, the card is probably bad. Replace it, and don't forget to format the new card before you start using it (the camera will complain if you don't).
• If the above procedure doesn't solve the problem, check for corrosion or dirt in the battery compartment. A pencil eraser can be used to clean dirty contacts. If there's visible corrosion (rust or greenish crud) it might be better to have the battery compartment cleaned by a professional as there might be collateral damage. Make sure to remove any debris before you put the batteries back in.
• Finally, if you're not having any luck after all of the above, start looking for a competent repair shop.