gas/hot water heater

I just purchased a home with Gas heat (forced hot water). I recently changed the thermostat to a programmable kind and it's been working for the last 4 days. Yesterday, when set the heat, it failed to come on. I checked the furnace, and the pilot light is lit, I checked the thermostat and all of the wires/connections are still connected securely.

I'm wondering if the pilot light is too low? And if so, how would I adjust it? Or how can I tell if there is a spark that is not sparking to ignight the flame to heat the water? If that makes sense, is that easily replaceable? (Whatever it is that causes the spark)? Sorry if this sounds silly- I'm not that familiar with furnaces.

Thanks