What happens if you buy a stock of a company and another company buys that company?

Stock would convert to a/the new symbol. Quantities of shares owned would be adjusted accordingly.

 

You don't necessarily get shares from the purchasing company. I've had companies buy out another company and payout the shareholders entirely in cash at the per-share buyout offering price. I've also had non-publicly traded companies buy a publicly-traded company and take it private, which also results in cash for your shares.