Yup, not a usual topic for mentoring or networking but certainly a key factor in how/why one builds community.
There are all kinds of definitions but this post is just about the one where you're surrounded by teams, friends or other communities and you still feel isolated. That kind of loneliness can last a day or a lifetime (or feel like a lifetime).
Loneliness can be self-imposed or be a matter of circumstance. It doesn't really matter the reason if one is feeling lonely.
Networks are not a buffer against being lonely. Friends are. Family is. Friends and families are networks and communities by definition but, when feeling lonely, sometimes strangers and acquaintances are easier to talk with - they won't necessarily push for action or challenge the facts of the story. These conversations are how work relationships can progress to friendship or just make things awkward. Measure the risks and make a decision that works best for you.
Basically, reaching out when feeling lonely is a great thing to do. If, like me, you have days where it's just easier to acknowledge being lonely and not reach out, that's fine too. We build communities in advance of a crisis, small or large, so there is help if we want it during.