It would be very unlikely that a juniper went from green & moist to brown and dry in a day. Juniper tends to stay green for quite awhile after the roots are actually dead - like a cut christmas tree will stay green for a month.
It is very difficult for some to keep juniper alive indoors. It really likes sun. Also it can be hard for a beginner to water a bonsai properly. If you have a photo you could post that, but from the description it does not sound promising. Check out the
mallsai and ideas about
Watering.
Don't give up, we all lose trees. You'll do better next time!
Regards,
Matt