It can take some time for your body to get back to normal after a miscarriage, especially if you were further along in the pregnancy at the time the loss occurred. Most women will have a menstrual period within four to six weeks after a miscarriage, but it may take as long as two to three months before menstruation returns to normal. If it's been less than that time span, try to have patience -- your body may simply need a little time to recover.
If it's been longer than a couple of months since your miscarriage and you have not yet had a period, call your doctor. Most likely, there's no serious problem, but a small number of women may develop adhesions or Asherman's syndrome following a miscarriage. Signs of Asherman's syndrome include having no menstrual periods, having cramping but little or no bleeding at the time of an expected menstrual period, or trouble getting pregnant and/or recurrent miscarriages.
Finally, if you have been having unprotected sex since the miscarriage, consider taking a pregnancy test -- it is possible to become pregnant immediately after a miscarriage even if you haven't yet had a period. Some women do not experience any delay in the return of normal menstrual cycles, and thus, ovulation may occur as early as two weeks after a miscarriage.
After a Miscarriage: Physical Recovery. American Pregnancy Association. Accessed: 30 Jun 2010. http://www.americanpregnancy.org/pregnancyloss/mcphysicalrecovery.html
Miscarriage. March of Dimes. Accessed: 30 Jun 2010. http://www.marchofdimes.com/professionals/14332_1192.asp