1997 Nissan Sentra Car won't start

For about a month (since the temperatures outside began to fall), it has been a bit of a struggle to get my engine to turn over in the morning. Then, about a week ago, I drove my car to work, parked it at the office for about an hour, then tried to leave. The engine didn't make a sound. I turned the key and all of the electrical stuff came on but the engine was silent. Is the problem the ignition switch or starter, maybe? Thanks, Kari