I've suffered from depression quite a few years in my life. One basic long-term cause I have found is my basic worldview - whether I see reality as fundamentally "good" or "bad". This takes a long time to improve.
But in the short term, I find my feelings strongly influenced by my ability to effectively respond to what life has brought along. If I set twenty goals and fail at all in a row, I tend to feel pretty low, whereas if I succeed then I tend to feel alright. When I advise students beginning research in chemistry, for example, I suggest that if they start to get frustrated to go "wash the dishes". This is helpful because washing the glassware is an activity where success is very likely, whereas chemical research can be fraught with disappointments easily beyond the ability of a novice (or even expert) chemistry student to predict.
What we are seeing here is a phenomenon called learned helplessness. Since it affects rodents it should come as no surprise that it affects us too. All it amounts to is a basic learning process that generalizes recent failure at a particular task to be a reduced ability to accomplish all tasks. Unlike rodents, though, we have the advantage of seeing through reason that the depressed feelings are unjustified, and acting out of reason in spite of how we feel. Students with difficult or impossible chemistry research assignments can still know themselves as competent chemists and worthwhile people.
It could also be burn out. How long have you been at the same computer-related occupation? I've been an application developer for the last ten years myself, but my life is full of other activities outside of work. Your desire to be outside may be your healthy need for a variety of activities. This would be a good time to go camping. It sounds like you are ready for a career change too, but you will need peace of mind and vision to lead you to the next one. That's my main advice - do what it takes to relax so you can reflect and know what to do.