Check that the window is unlocked.
Check for paint seals all along the inside and outside of the window frame. If the window is sealed by paint, use a putty knife to break the paint to free the window.
Check the window tracks, especially at the point where the window gets stuck, for any blockages, such as paint, rocks or other objects.
Insert the flat end of a crowbar as far underneath the window as possible. Gently pry the window up. If this isn't effective from one point on the window, try another.
Spray a liquid lubricant up and down both window tracks to help avoid future sticking events and aid the window in its operation.