If earth didn't spin on its axis but still revolved around the sun, how long would an earth day be?

If Earth was just revolving around the Sun, but was not spinning around it's own axis, than we would've had two big contrasts on the Earth, on the one side we would've had an endless day, and on the other side an endless night. This is due to the fact that one half of the Earth would always be directed towards the Sun, while the other will never be directed towards the Sun. The one part where there would be constant day will be very hot, while the other part with the constant night will be extremely cold.