Is it cybersquatting? Cybersquatting is when someone buys and sells a domain name with the bad faith intent to profit from another company's trademark. (We explain it in more detail, here.) For example, it would likely be cybersquatting if (1) the company set up a site that diverted your customers, (2) the company, in advertising the domain name for sale, mentioned that the potential buyer could trade off your previous traffic, or (3) the company set up a page with Google Ads listing you and your competitors. If the company contacted you, unsolicited, after buying the domain and offered to sell it back to you at a much higher price, that would likely be cybersquatting, too. It would not be cybersquatting if the company set up a non-competitive site for another company or if the company did nothing but set up a blank page. That's because there's no proof of bad faith intent. After all, it's not illegal to simply buy and sell domain names (without bad faith).
Should you proceed to ICANN? You are unlikely to get the domain name back by sending threatening letters. Most domain name dealers are not cowed by legal letters. They know you can only get the domain name back by (1) filing a federal cybersquatting lawsuit (probably way too expensive for you), or (2) by seeking domain name arbitration at ICANN. If you go the ICANN route, your filing expenses are, at minimum, $1500. It takes six months, requires a considerable amount of paperwork and documentation and may cost more if lawyers are involved. Remember, it's not enough to say that the company bought your domain name, you need to prove bad faith. And of course, there's no guarantee you'll prevail (although complaining parties prevail in 84% of the cases). Many people don't want the wait or the uncertainty of ICANN so they simply contact the new owners and pay for the domain name -- often forking over a sum between $2,000 and $3,000 because that's how much they would likely end up paying in time and money for an ICANN arbitration.