When I connect to net a message appears"modem can not detect a dial tone".I have reinstall the driver .

Try un-plugging all other phones from the line.
Check the modem is showing as OK in the device manager.

Assuming all else is good. The most common fix I have used is to buy a new modem cable.
They often stop working due to (dog chewing or) general wear and tear.
Alternatively, if you have a modern phone, knowing that the cable for that phone works try connecting it to the modem to see if it solves the issue.

You can try going in to control panel, system, hardware then device manager and go down list and click plus sign nest to medems and hightlight mosem and right click and choose uninstall and say OK.
Click OK at bootom cloas all apps and restart your comp.
On startup windows will reinstall a driver for your modem on it's own
Try to log on