2001 S40 Not Turning Over

[sharkiesteph]

I have a 2001 Volvo s40 that will not start. The battery is not the problem, but I can't diagnose it myself. When I went to start it this morning, it sounded normal, then I heard a "thunk." After that, it got quieter. It is not clicking, and I have 100% charge on the battery. When I turn they key, it just makes the quieter noise for as long as the key is turned. It does not sound like it will start. I was wondering what you think the problem might be, and what I will need to do to fix it. I don't know all that much about how it works, but my dad tried to check out the spark plugs. Could that be the problem? This is my only mode of transportation, so I'm very frustrated. I've only had the car since April of last year.