How come the value in my RAM of DS1307 RTC become zero?
Since you haven't posted the code or the circuit, we're reduced to making guesses or using our psychic powers. In this case, my guess is that you've wired it up wrongly, and the usual way to do this is to fail to connect something you're not using such as a reset pin.

In the datasheet I see "If a backup supply is not required, VBAT must be grounded" and also "When a backup supply is connected to the device and VCC is below VTP, reads and writes are inhibited. However, the timekeeping function continues unaffected by the lower input voltage".

My guess is that the device is in that state: the timekeeping continues to work, and the data is retained, but the device will not allow you to read it.