i made my modeling chocolate and stuck it in the fridge overnight like the video on YouTube said but now my chocolate's too hard.Is there a way I can fix this??

keep it in the microwave for just a few second like about 10 sec. Then break it into small pieces and then knead it till it becomes pliable. Knead small balls at a time and then mix them together to make a big ball.

After i tried this it became workable again!

however the oil came out of the chocolate and it became crumbly :(

Don't put modeling chocolate in the refrigerator. If a YouTube video told you to do that, it was wrong. You can, but you shouldn't. It's better to let it set up overnight at room temperature.