Turn Off Network Discovery

Go to System Preferences > Sharing and turn off File Sharing and other services that might allow people to see your computer when you are on public networks. At the bottom of the Sharing pane, on the right under Users, make sure that Everyone is Read Only.

Go to System Preferences > Security > Firewall and turn Firewall on (it is off by default). Leave this on at all times. Click the Advanced button at the bottom of the Firewall pane and check Enable Stealth Mode. (If the Advanced button is greyed out, click on the padlock icon and enter your admin password to allow changes to these settings.)

it turned off network discovery from all networks

Not sure what you mean here.

You can still join networks with these settings. They just help prevent your computer from being accessible by others on the same network. You can keep Stealth Mode and the Firewall enabled at all times.

You'll need to click on the AirPort icon in your menubar to choose a network to join, adding that network's password if necessary.

But other computers can't find my computer even in private networks

In these situations, turn File Sharing back on in System Preferences > Sharing.

You can also control access to specific directories by specified users in that preference pane.

Stealth Mode is a different animal:

When stealth mode is enabled, your computer will not respond to “ping” requests and does not answer to connection attempts from a closed TCP or UDP.

http://docs.info.apple.com/article.html?path=Mac/10.6/en/17133.html