Can I still use a Tagine that was outside for a while?
If all the tagine was exposed to is water and dirt then you should be fine as long as you clean it off and bake it to drive out any water. Tagines are generally unglazed on one side so will absorb water, if you don't dry it out it could crack when you try to use it. the process would be put it in a cold oven and turn it up to 170°C (350°F) for 2 hours, then turn the oven off and let it cool overnight with the door closed. The baking may tell if you have problems, if it smells nasty you'll probably want to get rid of it, if it's a normal clay smell you should be good.
If your tagine has been exposed to significant amounts of weed killer, bug killer, liquid fertilizers or other chemicals then it may have absorbed them in which case you'd want to think twice about using it. If there's been a couple of sprays around it there's nothing to worry about.