Earth is revolving around the sun in a balanced condition. Its inward moving tendency due to the centripetal force is equal to the outward moving tendency due to inertia of motion. If the earth loses some of its orbital speed, then its kinetic energy will decrease. Also, its tendency to move in a straight line will decrease. So, it will be dominated by the centripetal force which will cause spiral motion of the earth towards the sun. As a result, the earth will collapse into the sun.
Home Gravitation Reasonings What will happen to the earth if something took away its orbital speed?