I havent had periods for two months. ultrsound today showed endometrium thickness at 29mm. i am 41yrs. what could this be?

Hyperplasia: An endometrial thickness greater than than 5 mm is termed "endometrial hyperplasia." In women around the time of menopause, it can be caused by failure of ovulation which results in failure of menstruation and a build up of the uterine lining. At some point, this lining will collapse, causing heavy bleeding. You need further evaluation followed by appropriate treatment by a gynecologist ASAP.

Aging: As you age and approach menopause, it is possible that you'll skip periods and this might be the first sign of that as long as you are not pregnant. Other possible reasons for a missed period include a fluctuation of hormones related to thyroid problems commonly seen in middle aged women. Unless you're having heavy bleeding, I would continue to watch and track your cycles.