Is it a mk3 or mk4 Mondeo (2007 is the change-over)
The bonnet should "pop up" a little when you turn the key to the left, if its not doing anything catch could be jammed/ siezed
If you only have 1 key i would be careful not to put too much force on it - just got to use your jugement
Try turning the key while pushing the front of the bonnet down with your hand, this can relieve some of the tension on the lock mechanism.
this is a common problem on the mark3 if the above does not work it could be a broken rod which will need replacing see the first link here.