If you are used to starting payments directly on your terminal, and that is not working, you should check if "standalone" mode is enabled for your terminals.
Verify the settings of the terminal in the Customer Area. If standalone is not enabled for your terminal, follow these instructions to set it up. Afterwards, update the configuration on your terminal and try to start a transaction.
In case the keys of your terminal are completely unresponsive, try to perform a keypad diagnostic test.