Probably an egg shaped BB lodged in the barrel with 5 or 6 BBs behind it.
Step 1 empty chamber of all BBs.
Step 2 open action, fully cocked with lever extended.
Step 3 Hold gun upside down and tap stock on ground to dislodge the 5 or 6 BBs from the barrel, empty each one from the chamber as they come out.
Step 4 Use a thin long rod (wire coat hanger worked for me) to ram the odd shaped BB back down the barrel and remove from gun like the last few.
Step 5 refill and shoot.