It could be a Vocal cord paralysis. Most of the times the vocal cord paralysis do not have any cause, they ca be idiopathic! Wait for at least three months to see any improvement in voice as the opposite cord movement compensates for the paralysed cord and there should be an improvement in the voice.
If not, there are surgical options to improve her voice.
Next Steps
Three months observation if she is not having any problem in swallowing.
Health Tips
Voice therapy for three months.