For me - bonking is my heart beat in my ears so loud everything else sounds muddy, heart rate unable to come down, breathing unable to slow or exchange enough oxygen, everything starts to look bright & watery, and I feel like I'm gonna hurl. There is NO staying upright on the bike, and gravity becomes more difficult to overcome. It takes a Long recovery, and lots of fuel & rest.
Avoidance: eat enough, drink enough, get the electrolytes in, and don't push too hard or in too much heat. Listen to your body when you're riding, and get to know early indicators so you can compensate accordingly - bonking is Not something you can just Push Thru. I find it's most likely to occur in climbs, due to the extra effort required.
Recovery if you hit that point, and are alone: First and Always make sure someone knows where you're going & how long it should take. Have a cell phone on you for emergency assistance. Bring extra food & water with you. Get off your bike safely, but stay on/near the trail & visible. If you feel faint, focus intently on slowing your heart rate & breathing, keep your chest/lungs open, hold on to a tree or a sign post or something, but don't just lay down & decide that a nap would help. As soon as you can take food/drink w/out throwing up, do so - and try to consume more than you think you need or want. Once the world stops spinning and you can focus more than 2 feet away from your shoes, start to plan the safest way home & don't be too proud to ask for a pick up.