The concept of "midlife crisis" has been a staple of popular culture for years, and yet the best recent research seems to suggest that it might not exist. This doesn't mean that people -- your husband most likely included -- don't go through rough times in the middle years of their lives; it only means that there does not appear to be a classic condition unique to middle age, nor does arriving in middle age strongly predict the development of a "crisis."
On the other hand, depression is a well-recognized and dangerous disease condition that affects up to a quarter of the U.S. population at one time or another in their lives. Men appear to become more vulnerable to depression as they age. You can tell if your husband has depression if he has the symptoms of depression, which include:
• Feeling down, sad, or blue most of the day
• Losing interest in things he used to enjoy
• Sleeping too much or too little
• Eating too much or too little
• Suffering emotions ranging from low self-esteem to illogical guilt
• Feelings of hopelessness and helplessness
• Difficulties thinking or concentrating
• Fatigue or lack of energy
• Thoughts or plans of suicide
If your husband is demonstrating these symptoms, and if they are interfering with his ability to function, then he has major depression. But this doesn't address the question of why he has these symptoms, and this goes back to your question about a midlife crisis.
There is no way I can tell from your very brief question the degree to which circumstances in your husband's life are driving his emotions, and this is an important point. At one extreme, your husband may have one of the rare depressions that seem to come out of nowhere, meaning that his depression is out of all proportion to the circumstances in his life. This scenario becomes more common the more often a person has been depressed in the past. Said differently, each episode of major depression seems to sensitize the body and brain so that a person becomes depressed over smaller and smaller things with the passage of time.
On the other hand, your husband's depression might be a reaction to stressors and circumstances that would make almost anyone upset. Has he lost a job recently, or suffered a real blow to his self-esteem or health? Has he lost people close to him? All these factors can powerfully drive depression.
Your comment about him thinking the marriage is over and feeling worthless strikes me as especially suggestive. If the marriage is sound, and a source of comfort to both of you, his feelings are classic symptoms of depression. But sometimes people feel this way (i.e. marriage over and worthless) when they have done something they are ashamed of, but can't confess. Not knowing your situation, I have no idea which scenario more closely approaches the truth.
Although I can't give specific advice in this column, people struggling with depression are almost always best helped by seeing a trained clinician. In addition, from your question it does not seem to me that you have a clear sense of why your husband is feeling the way he is -- assuming that there is a "why" for his emotions. Often it can be very healing -- although also painful and not without risks -- to find a way to help him actually talk about what is going on. Coming clean, if there is something he needs to tell you that he hasn't, can be a remarkable healer.