Have you dated before u married?
if she is older than 50 she may have some changes in sexual life, but completely asexual???
anyway:
buy her a sexy dress , light candles, open a champaign, tell that she is the one ..etc and enjoy the evening.....
a lack of appreciation can lead to a bulimia, she needs to be loved in the first place, after u have accomplished your task, if u do love her!!!, u may take her to doctors.(u can share but not disclaim responsibility)
best