What you're describing sounds like secondary infertility. That diagnosis refers to a woman who was previously able to get pregnant and give birth to a child, but is now unable to conceive after trying for a year—assuming she's younger than 35 and has regular periods. (If you're older, you should wait no longer than six months before consulting with a specialist, and if your periods are erratic, you should see an expert as soon as you decide to expand your family.)
At least 10% of women experience secondary infertility, according to national estimates. It can be an isolating scenario, because couples who already have at least one child tend to get less support than those who are struggling to have their very first child. It can also be confusing: Why would you be able to give birth to one or more children without medical intervention and then become infertile?
There are actually a number of reasons why a couple who once conceived naturally would now be having trouble. For starters, your body may have changed in the years since you had your previous child. You could have developed endometriosis (a fallopian tube disease or blockage) or a hormonal disorder called polycystic ovary syndrome (PCOS). Any of these issues could make it much more difficult to get pregnant naturally.
Of course, you're now also older than you were the last time you got pregnant, and your ability to conceive gets harder as you age due to a decline in egg quality and quantity.
Your partner's health matters, too. He might now have a lower sperm count or sperm motility issues (meaning they can't swim as well). That's most apt to happen if he has developed a chronic disease, like diabetes. If he's having trouble ejaculating, that would also pose a problem.
Lifestyle habits also play a role in fertility. If either of you smokes or drinks excessive amounts of alcohol, it's going to make it much harder to conceive and have a healthy pregnancy. In women, smoking increases the risk of going into menopause early; in men, it affects both sperm quality and quantity.
Meanwhile, you should both take a look at your diet and weight. Good nutrition is important, and people who are overweight—or underweight—have more difficulty getting pregnant. Maintaining a healthy weight has a big impact on the function of your body's hormones, including the sex hormones you need to have a baby. And make sure your guy isn't taking any body-building supplements: Many of these contain testosterone, and if his testosterone levels get too high, his body will stop producing sperm.
The good news is that there are now a number of treatments and procedures that can help many couples who are struggling with infertility, including secondary infertility. A visit to a fertility specialist can help you figure out the best plan for you.