Janitrol Model Wh -100 I Don't Understand Why My Furnace Flames Will Not Come Back On?

I don't understand why my Furnace flames will not come back on? - janitrol model wh -100

I have an older model oven. It's like an elephant of enormous size. Anyway, I bought my house 6 years ago, and I know that this oven is about 25, 30th My fan is running, not just heating the air. My model is a packager and Janitrol gas. I really do not know what to do. And I am very afraid of what might cost to repair or replace my oven. If someone could help me it would really be a happy mother. Thank you!