I found this:
"The mood ring is filled with or a clear jem lays on a layer of a thermochromic element that changes colors with temperature change."
So it's possible that you are just the same temperature all the time, therefore the ring doesn't change colour.
But if it now won't change colour no matter what, I can guess what has happened.
The layers of the ring have either come apart somehow due to exposure to some element that destroys them (like temperature or chemical or solvent) or they have come apart due to some sort of adhesive holding them together disolving.
You wouldn't be able to physically see a separation like this, it would be too small a space to see with the naked eye.
I would test it by putting it in boiling hot water, it should change colour again, which might just mean it will take a bit more extreme temperatures now to change the colour, or if it doesn't change colour, you know it's best to put it in a keepsake's box for now.
I don't think you can fix it though. Sorry :(