Sound tissue will be tan-white at the tips and plump/turgid looking. If the roots look like this, then don't go any further, they're ok. If they are black or very dark brown and slimy/limp, they need attention. The sniff test - if they smell earthy, like a good garden soil after a spring rain (poetic, huh?) they're probably fine. If they smell sour or rotten, they need attention. Good luck!! Your "... a deep desire to learn." is enough to make everyone anxious to help. ;o) Al

My advice. You can choose your line of action. 1. Remove plant from the pot. 2. Remove old medium. 3. Remove sarongs (dry tissue on the p.bulbs). 4. Take a good look at your plant. From the look of it I would say it doesn't have anough roots to support it. The reason - old, decomposed medium and overwatering. Catt's medium must dry out between waterings. 5. Cut off all dead roots. Be careful! Some root look brown but they are not dead. In this case every root is precious. Squeeze them - if they are firm - don't cut. 6. Cut off all dead p.bulbs. On the picture I see some. Even if the leaf is green but the p.bulb tissue near the rhizome is dead - cut it off, it's useless. DO NOT cut off back p.bulbs if they are healthy! Plant use them for storage. 7. Sprinkle all cuts with cinnamon and leave the plant to dry for a coupple of hours. 8. Plant in new medium. Use dry medium or soak it and then let dry for a day before using. If you want to look like a real pro (lol) soak medium in rooting hormone + a drop of Superthrive + some fertilizer. 9. Find a place for your plant there it can get a lot of light but not direct sunlight. Put a big clear plastic bag on all that including pot. Sprinkle inside of the bag with water before. Try to keep the bag loose so it doesn't touch the leaves. Sprinkle the bag few times a day. DO NOT sprinkle plant. DO NOT water it. If it decide to live it will start to grow roots.10. Water ONLY then you'll see new roots with green ends and some white tissue after it an the roots. Let the medium dry out after every watering. Good luck. You can save practically every plant if there is still life in it. It's a question of how much time you are ready to waste on it.

Hopefully someone else will weigh in who is more of a Mandevilla expert, but IME they need very little water compared to most plants I've grown in pots. I let the soil dry out between waterings, though I don't grow in similar setting to you. I wouldn't bring it inside - it likes a good amount of light.

I am making recommendations on how to remedy the situation based on my growing experience with Crotons (indoors in pots 10 years). The OP obviously has little experience to judge the state, roots conditions, etc. we don't even know if the orig/new pot had a hole in it... it is necessary to hydrate the leaves quickly or they will drop, regardless of whether the rootball has water. BUT!He stated that he did not water the plant upon the transplant , only the soil below (that has no roots in it). It is possible that the soil has dried up somewhat. quote: I have replanted it in new soil (I suppose it's dry, it's the one from supermarket). I put lots of soil at the bottom, then just made it wet and then I put part of soil that was with the Croton (maybe like 40% of the soil that was on the plant before, otherwise i would have to cut too many roots) of it and then the new soil again on top. ---- SO, he did not water on top. Under normal conditions of warm temps Crotons take A LOT of water - especially the plant this size. Is the orig remaining rootball still wet? - stick a skewer/dowel into the soil to find out: OP needs to do that, before doing anything else. :) > down sizing the pot in my opinion disturbing this particular plant with more transplanting will worsen the situation. to increase humidity around the plant: it needs to be tented in a large plastic bag - that is the best remedy. running the hot shower (without the plant so as not to wet the soil further) and leaving the plant overnight there will help. I am wrapping up here.