If you've had your hair lightened and then decided platinum just isn't your thing, go back to your colorist and she can simply apply a darker toner to get the shade you prefer. Any good salon professional will be happy to adjust your color if you're not completely satisfied.
If you've used a permanent lighten only a little and still want it darker, try using a mild semipermanent or a temporary color until it's time for a touch-up, then use a darker product.
Occasionally, hair is so damaged that it just can't absorb color. In that case, your only recourse is to either have the damage cut off or wait for healthier hair to grow in. This might be the right time to treat yourself to a fun new hat.