If your NES doesn't work...well, in short, you're screwed. I've gone thru times where my NES failed me, and I know what you're going through. It sucks, doesn't it? Well, I'm going to take a look at a few of the ways to get your NES back in working order.
The first method, and the most obvious, is to get a cleaning kit. This will cost you no more than fifteen dollars. I bought a cleaning kit off of eBay and I now swear by it. Cleaning kits will work I would say about ninety-five percent of the time. The brand that I prefer, because it is the only one I have experience with, is Doc's Ultra 2000 Cleaning Kit, or something like that. It really works.
Then there is the poor man's cleaning kit...isopropyl rubbing alcohol on a Q-tip. After cleaning the cartridges, you take the dry end of the Q-tip and dry them off. This method has it's group of supporters; I, however, have never used it. If so many people like it, I'm sure it works, but I would imagine not as well as a cleaning kit. Use at your own risk.
This method is one that I have also used. Insert the cart just enough so that you can push it down and have it lock in place. Then, turn on the power. The game will work, more often than not. This method sounds odd, but it does work.
If these don't work, you may need a new 72-pin connector. These are available for a small price at www.i-mcm.com (type Nintendo in the search bar). For more information about 72-pin connectors, click back in your browser and then going to my 72-pin connector section.
The final solution is to get a top-loading NES. These will cost you a small fortune, so I don't recommend doing it if you don't have to. However, a top-loader is virtually guaranteed to solve your problems.
There is one method that I used to use but never again will. And I have good reason for it. I will never, ever, blow on the contacts of a cartridge. Why? Let me explain. The pH of your breath is under seven. In other words, your breath is an acid. The contacts of an NES game are made of metal. Over time, acids corrode metal. Blowing on NES games may get your games to work for that one time. But it will eventually lead to your cart failing. How do I know this? I called Nintendo and asked them why you shouldn't blow on carts. The woman gave me this very knowledgeable answer with no hesitation. She seemed to know what she was talking about, so I followed her advice.
Do you have a method of getting carts to work? Tell me. Make sure to put "Cart Cleaning" in the subject line so that this e-mail doesn't get lost in my busy inbox.