The last thing you want, when you come home to a stuffy house or when you are about to leave, is for the windows to stick. The sudden jerk of the pane, as you hit whatever is preventing the pane from moving along the track, can be frustrating, especially if you can't see what's causing the jam. Sliding windows are usually very easy to open, but they can occasionally have their issues.
Dirt and Debris
The bottom track of the sliding window can collect dust and debris, eventually either blocking the sliding pane or making the journey along the track rather rough. Inspect the track, and if you don't see anything you can obviously remove, try dusting the entire track. If that doesn't work, spray a silicone lubricant or other lubricating oil onto the track. Use only a small amount. If the blockage turns out to be something like a dried blob of paint, scrape that off with a paint scraper.
Even if the track is clear, you might have something physically blocking the pane. When you check the track for debris, you should be able to see if there's a thumb lock or dowel stuck in the bottom track, which would obviously prevent the pane from sliding. If you've just moved into a new place formerly occupied by someone into home security, you could find that they've added an additional lock to the top track. As with the bottom track, inspect the entire top track. Not only could there be a thumb lock, but the screws in the top track, if any, could have come loose.
Sometimes, sliding window frames are anchored to the wall by screws hidden in the portion of the track that is usually covered by the pane when the window is closed. Replacing these screws is another security tactic to thwart attempts to open the window by lifting the sliding pane out. Also, a screw might have come loose, jamming the sliding pane. Try removing the pane by lifting it up -- the lower edge of the pane should clear the bottom track -- and pulling the pane toward you. If you can't lift the pane up enough to lift it out of the track, you might have to call in a professional to find the screw and cut it.
Sometimes, you can pull an old sliding windowpane off-track just enough to stop it from moving midway along the track. Fixing this is a matter of lifting and repositioning the pane. You might want to actually remove the pane and see if any part of it has become bent or dented. Lift the pane up and out to remove it; to replace it, insert the top of the pane back into the top track at an angle, swing the pane so that it is all vertical and lower it into the bottom track.
Unfortunately, it is possible for the window frame to have become dented or warped for several reasons, such as the house and foundation settling. In this case, you do need to have a professional come out and replace the frame, and there's a good chance you'll have to replace the window as well.