Understand that sexual attraction is about power and confidence.
The more power and confidence you express, the more she gets attracted to you.
That's the simple way to go and you have dozens of ways to reconnect with your inner sources of power:
Start with career and ambitions.
Follow up with body power and energizing your being.
Next step, activate your social life.
There si more on that topic all over this site.
(You realize that the key goal of this site is to empower you in one way or another, right?)
What if you are a woman?
Most of the times, it has to do with getting too "comfortable" within your relationship.
You stop doing what made you attractive to him in the first place.
Ask yourself: How sexy are you right now?
Now, give yourself a mark between 0-100%.
0 = not sexy at all
100 = totally sexy
If your partner is no longer attracted to you, it usually means that you are losing this sexy edge.
Work on it! Get it back and you will feel his desire for you waking up again.