First, do they have kids...this changes everything and she may simply be exhausted. Esp. if she works outside the home.
Second, how do you get along on an emotional level? Does she LIKE you (apart from sex)? Do you laugh together? Can she tell you things freely? Do you criticise her or vice versa a lot? Sometimes, emotional well-being is connected to a woman's sex drive. If she doesn't feel satified emotionally, then she may not want to give sexually.
Hint: does she call you an assh*le (or its equivalent) w/any regularity? This would be a big sign...
For the record, I agree with Neo (!!), BUT only as a last resort. Basic needs shouldn't be denied anyon, hers or yours.
Just guessing; its hard to know w/o hearing her side of it.