If mars had water on , then what happened to that water

It's still there, usually frozen, underneath a layer of dirt or deeper underground. Mars polar caps are BOTH water ice AND dry ice - frozen carbon dioxide. Mars has enough water to cover Mars to a depth of 3 meters or more. The water is STILL THERE, just frozen most of the time.

http://www.huffingtonpost.co.uk/2012/06/…

http://en.wikipedia.org/wiki/Water_on_Ma…

http://mars.jpl.nasa.gov/msp98/why.html

Mars DID have liquid water in the geologic past. ANY one who says Mars has never had liquid water IS WRONG, because The geomorphology that proves that water was liquid AND IS STILL liquid on mars IS RIGHT THERE in the geomorphology of Mars. It SCREAMS LIQUID water. The scientific evidence IS THERE, no matter how much some people want to deny that evidence. They are WRONG.

Mars has never had water in liquid state.
There is frozen water within the rock / soil of Mars.

It is too cold at minus 50 Celsius for any liquid water to exist.

And it can never hold a dense atmosphere because its mass is only 1/10th that of earth and its gravity is too weak

There was gas when volcanoes were active. But the gases were being lost to space because gas molecules were moving faster than Mars escape velocity.

At the end of the volcanism to now, Mars has been just as we see it now.

The Martian atmosphere is too thin to allow the existence of liquid water ( for any significant length of time). So, unless Mars had a much much much thicker atmosphere at some time in its history, water never existed in any large bodies on its surface, ever. Keep in mind I am talking about lakes, rivers, or oceans that persisted for decades, centuries or millenia.
Water is present on Mars as ice and hydrates. Some has been lost to space.
-=-
A hydrate is a mineral which has water molecules in it as part of its crystal structure. Cement and concrete are both hydrates (or contain hydrates), so is gypsum (aka alabaster).