This is rare, but there is a chance your project's gradle offline mode is enable, disable offline mode with the following steps;
- In android studio, locate the file tab of the header and click
- In the drop own menu, select settings
- In the dialog produced, select "Build, Execution, Deploy" and then select "Gradle"
- Finally uncheck the "offline work" check box and apply changes
If this doesn't work leave a comment describing your Logcat response and i'll try to help more.