OK not necessarily a huge problem. Probably a clogged nipple vent. Question... are you SURE you poured powder down when you loaded it? You wouldn't be the only guy who has done this, so don't feel bad. I've actually done it more than once. "There are two types of folks that own sidelock muzzleloaders. Those that have dry-balled a load (means forgot the main charge), and those that eventually will dry-ball a load."
Clearing:
1) While in a safe place with the muzzle pointing in a safe direction, carefully use your ramrod to check to be sure the live load hasn't moved forward from the breech. Probably hasn't, but make sure. Then remove the ramrod from the barrel.
2) Remove remove the nipple, and you may need a safety pin as you make sure the hole at the base of the nipple where the spark from the cap enters the rifle, is clear, and the inside of the nipple is clear as well.
3) Using a piece of stiff wire, poke through the "channel" where the fire from the cap travels from the nipple attachment area, through the breech to the main charge. Be careful not to get the wire jammed in there or break it off. You are ensuring the spark has an open path from the nipple to the main charge.
4) Now some folks say to do this, others don't like it. Take a very small amount of powder. Just a few granules, like you might have in half of a quarter-teaspoon, and drop this down the hole for the nipple.
Replace the nipple, making sure it's all the way into the rifle. Your adding just a tiny bit of powder between the nipple and main charge is increasing the power of the cap and the spark from the cap.
5a) IF you're a prayin' fellow, say a prayer the rifle goes off.
5b) Place a cap on the nipple, and point the rifle in a safe direction, and attempt to fire. Hold it facing downrange for one good minute, if it doesn't immediately fire, as I've seen them smolder and then go BOOM when this is tried.
6) IF the rifle went BOOM, clean it completely, and don't forget to do a good job cleaning out the nipple and where the nipple attaches as you added some powder there.
IF it fails to fire, you can repeat the procedure, but it's probably a bad main charge, and you will need to get a ball discharger that uses CO2, or a ball puller that uses a screw, or some folks use an attachment and use a car grease gun to get a main charge out.
Good Luck
LD