I'm fairly certain the spring is there, but will double check when I get home.
The other possibility is the bit of plastic on the back of the cyclone handle that the catch clicks onto has snapped off.
If you sit the cyclone in position, then pull forward slightly and follow back to where the catch would locate on the back of the cyclone handle - is the bit of plastic that the catch latches onto present? Thinking about it, if you have a clear handle you might be able see straight through to where it attaches anyway.
If this is the problem you need a new cyclone top or just the handle part if you have a torx screwdriver to undo the two screws.