Share prices:
If a company is on the stock market it is owned by the people who own shares in the company.
Say there are 1 million shares, and each share is worth �50, the company is then worth 50 million pounds.
If the share price stays at that, or even goes up, people will want to keep the shares, and maybe buy more if possible.
Now suppose the company has a wobble (profit warning, fire at a factory, ships a product that does not sell etc).
Confidence in the company drops, share price drops to �40, and all of a sudden company is only worth 40 million.
People get worried, start selling shares, nobody wants to buy them, price drops even more, now only worth �20. Value of company plummets.
Company needs to borrow money to get over crisis, but nobody will lend them any money (too risky). Share price drops to �10 then �5.
Company is in big trouble.
People cannot sell their shares quickly enough, nobody is buying their shares, company is now worth almost nothing.
Panic sets in, people laid off, parts of company sold off to highest bidder.
Company goes bankrupt and is sold for �1 to anyone who will take on the debts.