What: Shares of industrial equipment maker Caterpillar (NYSE: CAT) saw its stock drop 15% in September as investors started to realize that commodity markets could be in trouble for some time.

So what: Commodities like gold, silver, copper, and even soybeans have plummeted in price this year and Caterpillar is starting to feel the pinch. The company's revenue fell 6% in the second  quarter and with China's weakness of late and commodities continuing to fall there's worry that a recovery isn't in store any time soon. 

The real hammer fell on September 24 when the company announced it would cut 10,000 jobs through the end of 2018, including 5,000 between now and the end of 2016. Cuts like that show just how pessimistic management is about the next couple of years. 

Now what: This is what happens with cyclical stocks. They do well when the economy is booming and when it takes a turn south their operations and stocks can take a big hit. Caterpillar was one of the beneficiaries from the boom in commodities and the global economic recovery over the last six years, so now it's taking a breather.

Some of the things investors are worried about are very valid for Caterpillar. Demand from Asia, and China in particular, has been weak and we could be in for a long slowdown there. Commodities may also not reach the heights they were once at now that the economy is recovering and inflation doesn't appear to be a big problem. So, mining may be a bad market for equipment suppliers in the future.

But long-term I think Caterpillar has a strong business and has weathered these storms before. It's not cheap at 17 time forward earnings estimates, but weakness in the stock should be seen as an opportunity for investors with a long time horizon.