I own a 2002 volkswagen diesel (Jetta). It would not start, so I figured it was the battery, replaced that battery, it started right up, the next morning it would not start, what could be causing this? I've checked for trunk light being left on, that is not the problem. Any suggestions?

See if this link gives you any ideas:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235560