Could be a couple of possibilities. The big thing is to determine exactly what is no longer working. If you have a well-labeled panel, this will be a big plus.
Check and determine everything that is not working. Try to determine if this is just one circuit (if it is, it sounds like it is pretty well loaded) or if it is multiple circuits.
If you seem to have multiple circuits out, then the other answers are probably correct. You have lost one of your 120 volt power legs. It could be the utility transformer, their connections, the meter / meter base, or your main breaker. If this is the case, call the power company and let them check their stuff first. They normally do not charge for this, since maintenance is their responsibility. If their equipment checks out fine, you will need a licensed electrician. You probably have a main breaker problem. I'm sorry, but I do not recommend troubleshooting / replacing a main breaker as a DIY job. Please call a pro for this.
If this appears to be just one circuit, then you have a different problem. First, verify that there are no tripped breakers. Be aware that it is not uncommon for a breaker to trip and have the handle stay in the 'on' position, giving the appearance that is is still set.
If all breakers are on and working properly, here is a method to find the problem. Not 100% guaranteed, but works almost all of the time. If it doesn't, you will be stuck opening each receptacle on the circuit until you find the problem.
Turn on all of your non-working lights and plug lamps into any non-working receptacles. Make sure that any computers, or other sensitive electronics are not plugged into this circuit. Take a lamp or an extension cord around to the all of the receptacles on this circuit. DON'T rule out the working receptacles either. Try them too. Plug it in and wiggle the plug (hard enough to wiggle the receptacle too). You will probably find one where doing this suddenly causes your non-working lights/receptacles to come back to life. This will be your culprit.
A lot of homes are wired with receptacles using 'stab-in-the-back' connections. These are actually still in use and are deemed safe for installation. The problem is that they have a terrible failure rate. The term 'stab-in-the-back' also applies to what they do to you later. Any replacement should have screw terminal connections.