At the minimum it will end your recreation for the day and at the worst it will end your life. Both of these are excellent bits of advice. The first thing you should do is take a look at the ice before setting foot on it. Clear blue or blue black colored ice free of snow, air bubbles or debris is the strongest. Ice that is white, brittle, mixed with snow, or filled with air bubbles isn't as strong and won't hold as much weight.
Ice is actually very complex.
Factors including water currents and depth, snow cover, and temperature all effect how strong ice is. It takes two to three weeks of temperatures below freezing to make deep lake ice capable of holding up a large amount of weight. New ice is stronger than old ice. If you're not familiar with an area talk with local officials, bait shops, or check with the local police department on ice thickness. Of course looking from the surface, you won't be able to gauge how thick the ice is.