Different things can happen. Light helps algae to grow Light can increase the temperature of water if they are too high of wattage. Your fish tank should only get about 8-12 hours of light per day. I keep mine on my 60 gallon about 12 hours so my nocturnal organisms can have a decent feeding and exercise time.
Um, nothing, really. The fish will just be, you know, in light, all the time. Unless the lamp has like a really high wattage and is going to turn your Goldie into fish sticks, than you should be good. However, you're really going to run up your electrical bill if you just keep the light on 24/7 - so try to turn it off when you can. Hope this helps!