Do you have a double-hung window that has been painted shut? If you do, you’ll find you can usually get it working again by cutting through the paint that is sealing the sash around its perimeter. Try this:

Use a utility knife—or, better yet, a special little tool that looks like a serrated metal spatula called a window saw (available at paint and hardware stores for about $10)—to cut through the paint all around the movable sash. Be careful not to leave any areas uncut both inside and out and also take care not to damage the painted surfaces. 

Make sure the window is unlocked. Also make sure that there are no nails or screws securing the sash to the jamb; if there are, remove them. Then try to force the window open with abrupt force, using the heels of your hands.

If that doesn’t work, try to free the window with a wood block and a hammer or mallet, but be careful not to jar the window so hard that you break the glass! Once you get the window open, brush away the loose paint and debris. (For more information, also see How to Free a Stuck Window Sash.)