Clicking button locks mouse, doesn't activate button. how do I fix this?

Hey guys, I have a piece of code that locks the mouse and hides it when the player clicks the screen, but when I try to click a button on the screen after the mouse has been released, the code that locks it must override it and locks the cursor instead of activating the buttons functions. the piece's of code in question are

update(){
      if(Input.GetKey(KeyCode.Mouse0)){ //locks mouse
         Screen.showCursor = false;
         Screen.lockCursor = true;
         }
      if(Input.GetKeyDown(KeyCode.Escape)){ //releases mouse
        Screen.showCursor = true;
        Screen.lockCursor = false;
      }
} 
and 

function OnGUI() {
      //button I wish to click.
   if(GUI.Button(Rect(0, 0, 240, 40), "(Press escape to free mouse)\\n"+
                                       "(Click here to return to main menu)")){
       Application.LoadLevel("MainMenu");
    }
}