This is a very hard question to answer. It depends also on how old you both are as marriage problems go in phases. So, I'll break it down for you:
If you are fairly young and married, then the magic can go out of a marriage and it takes a lot of hard work to have a good balancing act in a successful marriage. There will be some bumpy roads to go through such as possibly cheating (one partner or both) or just boredom where one or the other stays out after work and doesn't take the time with their partner. Missing important dates, getting odd phone calls at different times of the evening; secrecy, not talking, drop in sexual relations.
In the 40s+ both men and women can go through menopause. Men's use to be called "midlife crisis" but it is proven it's just plain old menopause and although they don't go through near the hormone changes women do, they can get moody, lethargic, feel they need to recapture youth (may have an affair because of it) and are angry in general and depressed. Women can certainly get cranky, moody, burst into tears or burst into rages without the slightest hint to the guy, depressed (often confused with bi-polar) and often will ask if she's still pretty, fat, or does he find her attractive. Both partners can feel like they are missing something in life and have huge urges to leave the marriage in hopes of something better.
It's really time to just sit down with her and tell her how you feel and certainly ask how she feels. If your sex life isn't great, then don't fret, because any age can suffer from this depending how hard either partner works, shift-work, long hours, or, even medications can cause low libido. Perhaps both of you simply need a vacation. If you can't afford much go on a mini vacation. If you have kids get family or a good friend to look after them for a long weekend and go some place together.
Communication is the best solution to any problem in a marriage. Without it, we only can second-guess what the other person is thinking and that can cause a lot of trouble. We live in a very stressful and fast-paced world and sometimes a person is just plain dog tired.
Good luck Merry Christmas Marcy