Shredder

Hi,

While trying to fix my bike i have come across a problem:

The rear derailler will allow me to shift up but not down. What have I done or is the rear derailler worn out?

Does the cable need tightening? I have tried tightening it. In top gear it is tight but in the lowest there is no tension yet the rear mech wont shift down? are the springs worn out. (N.B. the bike is a few years old)

Thanks for any replies.