It's a feeling of desperation when you're separated from the one you love. You feel so empty inside. You can't sleep. You can't eat. All you can do is lay on your bed and stare. You're always staring off because the reality is that loneliness is your mind's attempt to be somewhere that it's not. You can't focus on anything but your own emptiness. Loneliness is needing someone but knowing that there isn't anything you can do about it. The only solution is to be reunited with whatever it is you long for.
The man woke up, alone, the sudden realization that his love was not next to him overwhelming him with loneliness.