Steam allows you to play games offline (off the grid) or offline without the need to connect to the Internet. If you're ready to access the online Steam community (online) or online, you can do this directly through the Steam menu on Windows, or by editing the Steam registry file in the Finder on Mac OS X.
Step
Method 1 of 2: Enabling Online Mode in Windows

Step 1. Launch the Steam application on your computer

Step 2. Click “Steam” located in the top left corner of your Steam session

Step 3. Select “Go online”

Step 4. Click “Okay” when it is notified that Steam must restart to enter online mode
Steam will restart, enter online mode, and exit offline mode.
Method 2 of 2: Enabling Online Mode on Mac OS X

Step 1. Launch the Steam application on your computer

Step 2. Click “Steam” located in the top left corner of your Steam session

Step 3. Select “Go online”

Step 4. Click “Okay” when it is notified that Steam must restart to enter online mode
Steam will restart, enter online mode, and exit offline mode.
Continue with the following steps if Steam doesn't go online

Step 5. Open a new Finder window on your Mac OS X computer

Step 6. Type “users” in the Finder search field

Step 7. Double-click your Steam account name

Step 8. Open the “Library” folder

Step 9. Open “Application Support”

Step 10. Open “Steam”

Step 11. Open the file with the name “registry
vdf” using TextEdit.

Step 12. Look for the text that says, “offline 1”

Step 13. Change the number “1” to “0”

Step 14. Exit TextEdit and select the “Save” option at the prompt

Step 15. Close and reopen the Steam application on your computer

Step 16. Click on “Account” located in the top left corner of your Steam session

Step 17. Select “Go Online”
Steam will exit offline mode and go online.