My 97 civic has 226,340 miles on it. My check engine light just came on? What might cause this?

It could be many things:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post325940

Attach a code reader or scan tool to the Data Link Connector, located under the dash, and read the code or codes thrown.