Have your owner's manual on hand in the event a minor adjustment does not resolve the locking problem.
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Disengage the window lock, and lower the upper sash. Inspect the channel in the upper frame to ensure there are no obstructions. Look for a bent channel in the upper part of the frame. Straighten any bends or remove obstructions that would prevent the upper sash from fully seating inside the upper channel. Look at the top of the upper sash and remove any debris.
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Push the upper sash fully into the upper channel. Attempt to engage the window lock. If the window still will not lock, lift the lower sash and inspect the lower channel as you did the upper channel. Close the lower sash and try to lock the window.
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Tilt both sashes into the room as if you were cleaning the sashes. Pull the end locks toward the center and tilt the top of each sash into the room. Push both sashes firmly back into the window frame. This will realign the window sashes in the channel.
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Inspect the window balance of your window sash. Sometimes the balance will disengage, causing the window not to lock. Refer to the owner's manual for your window because access and adjustment to the window balance varies.