velux blind doesn't closing anymore(manaul rolling blind)?

I've had blind for velux windows,it was worked perfectly but now,i've got problem them blinds weren't close anymore,every time i pulled down it always rolled back to the top slowly and opened itself completely after.is anyone know how to fix this problem??
i rang the place where i've got them blinds installed.but their never cames out to fix them..and i would recommend not to get blind from this place(BRADY BLIND) in DROGHEDA, no service after sell.
and wanted it in their way,by not to listened to the customers...