My 2011 maxima sv is losing power when driving, i stepped on the gas and the car didnt accelerate, the rpms and the speedometer didnt move either. And the next day the battery was dead, so if the alternator is good then what could the problem

 

Dead battety can be caused by many problems.
First - charge the battery and measure battery charging (most common issue)
Second - if battery charging is fine - measure current consumption. Sometimes battery can be drained by broken steering wheel position adjustment motor, cabin light, etc.
Finally - if current consumption is ok, measure battery condition