What happens when the MOSFET isn't conducting, circuit-wise?
Note that M1 is N channel while M2 is P channel, although they both invert from gate to drain in this configuration.

With Vin at 0, M1 is off.  With the gate of M2 at 0, M2 is presumably on.  I say "presumably" since you have provided no specs on Vdd and the required gate voltage for M2 to be on.  I'm assuming these have been arranged such that a P channel FET with source tied to Vdd is on when its gate is at 0 V.

From above, replace M1 with a open switch and M2 with a closed switch.  Vout is now obvious.  Draw it out as open/closed switches if that helps you see it.