Apart from calling out a professional carpet cleaner there are definately things you can do to resolve this common carpet cleaning scenario. Below is what I would do. I do not recommend you do this. I would test on spare carpet first before I made any attempt at removal.
I would firstly decide what the stain on the carpet is. Once I had done this(they are usually black marks that do not dissolve with water) I would use something like methylated spirit or wd40. I would apply the product to a white terry towl type cloth and dap from the outside to the inside of the stain.
Once I had removed the oil from the carpet I would use water and kitchen towels to rinse the area.
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