Have a go at it with the Levels (Image > Adjust > Levels). Under the histogram, you'll see three sliders - a black one on the left, a grey one in the middle, and a 'white' one on the right. These control the overall lightness of the image. If you slide the black one to the right, the image will get darker. Try adjusting all three in small increments and see what you come up with. At the bottom right of the Levels dialog box, there are three 'eyedropper' tools. You can set the range in the picture with these. Choose the left-most one (black) and click on any spot of black in your image (if there is one). The middle dropper should be clicked on a mid-grey in the image, and the right-most one on pure white. By doing this, you're sampling those particular colours and allowing Photoshop to work out the tonal range of the image. If it all goes wrong, hold down the Alt key and the 'cancel' button will become a 'reset' button. That takes you back to how the image was when you first went into the 'Level' option. Good luck :)