It's a horrible, horrible experience.
Usually happens in hot weather - dehydration being a contributing factor.
It's the total and sudden debilitating loss of energy and onset of fatigue and weakness, the result of the stores of glycogen in the liver and muscles becoming depleted.
When this happens in hot weather, it feels like your entire body is going to melt into the sun-softened black tarmac, every slight incline becomes a major climb, the slightest breeze becomes a full on gale, and your mind sinks into total and utter blackness of despair.
As you become more experienced in cycling, and if you cycle enough you're almost certain to experience the Bonk - even (especially) racers; even pro riders such as Mark Cavendish have suffered with this, you'll get to know your body and pick up the early warning signs. Cycling in hot weather, such as today, is a high risk day if you don't drink enough fluid. On hot mornings, it's easy to skip breakfast - but that boost of carbohydrates is essential, as is eating a good mix of carbs, proteins and fats the evening before.
A couple of weeks ago I did my 55 mile route. Usually this is easy enough to do, elapsed time being circa 4 hours, riding time 3.5, with 2 x 15 min stops for refreshments. On this occasion it was warm - not baking like today - and windy. I got to my second stop, in Amersham, and realised I hadn't taken enough cash for drinks AND flapjack at Costa Coffee, so I went for the drinks. I'd already felt a twinge of hunger in Bourne End, some miles before Amersham, and thought I was going to suffer on the way back.
I did consider shortening the route, but thought WTF, let's just do it... and I was OK actually until within 500 pedal strokes of home, I tried "sprinting" up a slope.. and the legs just refused to go - no zip or zing... I sat up and ground my way to the top of this pimple - pace for pace being matched by a young lad still on stabilisers, his legs whizzing around in a blur! haha!! Bless him... quite comical really, me being all togged out in racing gear...
Not quite the bonk, but my immediate stores of energy were depleted - and even if it was the dread Bonk, I was within falling over distance of my house... but had this happened with 20 miles to go I'd have really struggled to get back. Today I did the same route, back by midday to avoid the heat of the afternoon, and it was a breeze... just by way of additional info I weighed myself before and after the ride. 10st 10lbs before, 10st 7lbs after, even though I'd consumed over a litre of fluid on the way around!
To prevent this form of bonking, stock up on carbs the day before and ensure your breakfast has a good carb base - porridge for example. Eat carbs on the way around, sports drinks are formulated to give a dose of carbs as well as proteins and caffeine is included too as current theory is that caffeine, though a diuretic, speeds up the processing of carbs into energy.
Post ride you should eat something - low GI carbs are apparently favoured - but I really struggle to eat anything for up to a couple of hours after a ride. I can just about manage yoghurt with fruit and mixed nuts. For four hours after riding, the body will convert food into energy at a much faster rate than usual, for recovery purposes. If you are going to ride the next day as well, this post ride eat is therefore really important.
I have nothing to say about any other form of bonking on a bicycle, other than take extreme care ;)