First press <Esc> twice. If a command is in progress, the <Esc> key cancels it. If you are in insertion mode, the <Esc> key puts you back into command mode. If your terminal beeps or flashes when you press <Esc>, it means you were already in command mode.
If the screen is unreadable, press <Ctrl>L (or <Ctrl>R on some terminals) in command mode to make vi redraw the screen.
If you still cannot read the screen, either your terminal is set up incorrectly or you are editing a non-text file. Type :q! to exit without saving the current file. All the changes you made since the last save operation are discarded.