DIAGNOSIS The automatic cord rewind mechanism uses a spring. Over time, dirt and dust get into the housing, or the spring itself can fatigue or become misaligned, making it sluggish.
FIX Unplug the vacuum first. Hold the cord plug and manually wind the cord back in slowly and evenly, listening/feeling for grit or binding. Often, just doing this a few times helps redistribute things. While the cord is out, use a vacuum attachment to clean around the cord entry point and the recess where the plug sits, removing any visible dust or hair.
IF THAT DIDN'T WORK The issue might be internal. The C3's rewind mechanism isn't the easiest for users to service. If it's truly jammed or the spring is broken, it usually requires a technician to open the canister housing and access the rewind assembly.
HEADS UP Never yank the cord out or let it snap back violently. Guide it out and in with your hand. This greatly extends the life of the rewind mechanism.