No there is not an on-screen keyboard.
Why don't you use your keyboard to input the Wi-Fi password?
If your keyboard is an Apple wireless keyboard, then it is a Bluetooth keyboard. In the event the keyboard will not connect to your computer (you type and nothing happens!!) then simply follow these directions: Troubleshooting wireless mouse and keyboard issues - Apple Support
Once you have the keyboard connected and working then simply input the Wi-Fi password and you are done.
If you still have trouble then I strongly suggest you contact AppleCare and they can help you through the problem. If you don't know the phone number then please click https://locate.apple.com/country to get started.