My 1999 ford taurus was having trouble shifting into 3rd gear. I turned it off and left it alone for 2 days. When I drove it again it seemed to work fine. I wanted to check the fluid levels but can not find where to. My question is: what could be wrong? Is it transmission fluid, or is my transmission going out, or is it just a bad day?

As the owner of a 2000 Ford Taurus, I can tell you that the ATX is the weak link on these cars, just like on all FWD cars. At a minimum, the pan should be dropped and cleaned, transmission filter replaced, and refilled with Mercon V every 30,000. If you haven't been following this schedule, I would get it done ASAP. Look for the yellow dipstick, located between the throttle body and the firewall on the driver's side. Here's how to change the fluid yourself:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-5.html#post552241

There are many other helps for your Taurus in the Sticky--I know they work, because I wrote them up, after doing the jobs myself.