I2C doesn't working

I am using a MCP4725 DAC from Adafruit with ARDUINO UNOadk and I connect my SDA and SCL. It worked good, till today. But suddenly it stopped working , I checked my SDA signal it seems that doesn't a signal

What's exactly the problem

There is no way to know what is the problem from what you have told us.

When you say

how did you do this exactly?

it's almost as you're calling your mechanic and say:

My car used to work until recently and it worked good but suddenly it stopped working. I checked the tire pressure but could not get a reading.

What's exactly the problem?

do you think your mechanic will be able to answer?

Just load the I2C scanner sketch on the Arduino and diagnose from there. This may happen when pull-up resistors are not used or too high of a value. Could be a temperature change which affects a solderless breadboard connection (or cold-solder joint.)

I loaded and I found it

Thus, the uC can "see" the sensor over I2C. And you really have not changed the sketch?
If you have another I2C sensor, it would be helpful to do a part substitution.

The problem is that when I restart my board so many times it works fine but sometimes it stopped working by itself

Intermittent problems are the worst. Double-check values of I2C pull-up resistors. If using solderless prototyping, jumpers are always suspect.

I 'm using 4.7KOhm resistors for pullups I2C with 5V

Maybe try one if Adafruit's "example" sketches. If you do not have a spare I2C device, you really are at at the mercy of luck trying to find intermittent issues.

If using jumper wires, replace them one-by-one. If soldered, disconnect power and use a decent VOM/DVM to measure the resistances.

Not everyone can afford the luxury of 2-of-each part, but with substitution, it is the best method.