There are two opposing schools of thoughts about this situation.One is that a person can learn to love the partner again by focusing on what is loveable about him or her, what originally turned them on, or what might be changed that might reawaken love and desire.
All humans are sexual creatures and sexual expression takes many forms. The one who usually initiates stops doing so maybe because of frequent rejection, and the other, who certainly could initiate when ready, doesn’t. "I’m in love with/infatuated with someone else.” This does not necessarily preclude sex with one’s regular partner. I have counseled individuals and couples in both roles in the above situations – some with mutually acceptable outcomes, in some cases not.
Hearing this from a figure of authority like a physician or a marriage counselor can often make the difference. The sexual drought continues and, quite commonly, nobody brings the topic up until it becomes critical to one or the other. Many people have loved more than one person simultaneously. Every coupled relationship is different and contains individuals with varying degrees of commitment and flexibility.
In any case, we look first for a physical cause which can often be improved upon. Is it really a loss of interest in sex itself (does he/she masturbate or fantasize, for instance) or is it a loss of interest in the partner? “My partner no longer turns me on.” This is a more difficult situation but not impossible.
The complainer usually gives a “reason” such as the partner’s weight gain or unwillingness to engage in the type of sex s/he prefers.
So perhaps the problem isn't so much "recharging" the relationship as bringing one person up more than the other?