This really is a common trap. The Japanese call the extreme version "Hikikomori", but it seems to exist in most developed countries. You could look at it as a specific type of agoraphobia, one which mostly affects young men when entering adulthood, though of course it can last for years.

Gradually work yourself into a habit of leaving the house every day for at least a few hours, and doing something. Make it something fun, and visualize the benefits. Go to a matinee or something. Once you've regained some social skills (these have likely atrophied somewhat), get a job (one which requires you to leave the house every day). Just doing this much will likely make you much happier. I base this on the period of isolation I went through when I moved to a new city, but before I started work on my job and made friends. I started becoming withdrawn and depressed and stopped taking care of myself, which started a vicious cycle of low self-esteem and avoidance behavior. I just felt exhausted and used internet/video games to distract myself from my ever-growing problems.

The cycle broke when I finally had to pull myself together to start working. Just seeing myself cleaned up and well-dressed for once cheered me up. Being outside cheered me up. Having conversations where I didn't just (speak as little as possible, nod, avoid eye contact, etc.) and was seen as likeable made me fear social situations less. I started making friends, had more energy, and things have gone great for me since then. It is usually possible to come back from this.