I've suddenly lost consciousness a few times in my life, some due to a fainting condition, some due to drugs, some from being knocked out, and some from being choked - it's important to disguish the different types of forceful loss of consciousness.
Fainting - You get lightheaded, as in, your head physically feels light, sometimes to the point it feels like it has a "negative weight" and wants to float upwards. Your vision rapidly starts going black from the outside inwards, the world becomes blurry at the same time & you will stop being able to hear anything aside from extremely muffled sound and sometimes a deafening whitenoise / high pitch ringing. Your body also starts to feel tingly. Your limbs are movable but rapidly stop responding to inputs. This all happens within about 15 seconds, you can be unconsious for up to a minute. Waking up is very much a disorienting experience, depending on how prepared you were you may have fallen over and hurt yourself, be prepared for pain.
Drugs (what an overdose will be like) - Your head feels heavy, you physically don't have the strength to move it, you will also barely have the ability to keep your eyes open - breathing if possible will be done "automatically", that or you will be gasping or not even realise you're not breathing. Your vision will do the same as when you faint, but on a much faster rate depending on what quantities and type of the drug you took. Your hearing will go in-and-out of being clear to being muffled, to there being absolutely nothing. This can happen repeatedly (10-15 times) over the course of half an hour or longer, with each blackout lasting for 10-15 seconds. Waking up after it all ends will leave you feeling like shit for hours to days afterwards, with zero energy and a big headache. You will probably also throw up during and after this entire process.
Being knocked out - It's instant. You go from being conscious to not instantly. Waking up you will be in pain and heavily disoriented, you might throw up, it's unpleasant. You will likely have been unconsious for 5-10 seconds at most.
Being choked (air choke) - Unpleasant as you will be trying to gasp for air the whole time, you will have a pain in your lungs from CO2 buildup, and will slowly start to experience the same blackout process as fainting.
Being choked (blood choke) - Same as fainting.