How the price of stock is regulated in stock market?

price of stock not "regulated"

there no regulatory body deciding price of stock should be. sellers post asking price, buyers post bidding price...when buyer matched seller transaction occurs. market place of buyers , sellers controls share price.

fewer buyers means share price falls....fewer sellers means share price rises buyers try entice sellers sell stock them offering more money per share buy shares.

basis of supply , demand product.

share price dropped 20% in example because of bad news...a bad financial report released, ceo accused of fraud, drug in development failed approval sold, explosion @ plant shuts down production while, maybe 2008 bottom falls out of market in general due financial crisis....something that. 20% drop in day quite serious...normal fluctuations less 2%

seller @ price may or may not suffer...if bought shares @ 300p sold @ profit....just not big profit day previous. if bought shares around 500 or suffered loss of money when sold.

buyer has not suffered @ all....at least @ point. got shares 20% cheaper previous day
.
if share price rebounds in black immediately,
if price falls further anticipated further drop if knowledgeable in purchase , perhaps buy more or hold stocks until price rises (as expects in future.)

if buyer had no skills , bought because of large price drop (many this) trying catch falling knife , lose money in short term...perhaps long term (depends on seriousness of news)

what mean that,in opening of market stock x trades in ₹550 per share. @ closing of market stock trades in ₹450 per share. there decrease of ₹100 per share. "1.how price came down ?"
2.how seller suffer this?"
3.how buyer suffer this.

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