The Girdle of Melian is no longeer working because (as I recall) at Thingol's death, Melian became distraught, and let her power lapse, in effect destroying the Girdle. That was my impression.
Melian created it by using the power that she had.
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But what was its effect on foes?
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I believe that the foes just couldn't get through to Doriath.
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Against whom was it raised?
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Servants of Morgoth and unwelcome visitors.
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How long did it endure the attacks?
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Until Melian let her power lapse.
Please note that I haven't read the Silm recently, and I am working from memory. I am answering according to the impressions I had from reading the book.