The standard treatment of a missed miscarriage is a procedure called a dilation and currettage (d&c).
In some instances, medication can also be used and there is also the option of waiting. In a british study of 85 women who chose to wait, only 25% miscarried completely on their own. 43% had an incomplete miscarriage & the rest opted for a d&c. Talk to your doctor about your situation & options.