Tubes tied 5 years ago my doctor said that it may have failed but couldn't tell on the ultrasound no period in 4 months and could i be pregnant?

Could be: Your doctor is correct. Ultrasound alone cannot tell if the Fallopian tubes are closed or not. That does require different techniques. But it should be able to tell if you are pregnant. Discuss that point with your doctor.

Doubtful : Have you had a urine or blood pregnancy test. If those are negative, you are not pregnant.

It is possible.: The most common cause of missing period is pregnancy. Even though the failure rate of tubal ligation failure is generally quoted as 1 in 1000, it does occur. The first thing you will need is pregnancy test. If it returns positive, your doctor needs to make sure that the pregnancy is not an ectopic pregnancy, an abnormally placed tubal pregnancy which is increased after tubal ligation.