1. See a doctor - you may in fact be depressed. Failing that, talk to family, partner etc, people who actually know you. If they think you are depressed, you probably are.
2. Is it existential angst? Wondering what else you could be doing instead of what you are doing now? The grass not only looks greener on the other side, there are so many lawns to look at.
3. Ask yourself what you really want - if you want something else, change. You have 1 life, so live it. If being cooped up in an office most of the day, feeling disassociated from the world around you just isn't making you happy grasp the nettle and change your job.
4. Switch the laptop off and go outside and get some sunshine.