Unfortunately you haven't given us any information about your system. Therefore, all proposed solutions are a look into the crystal ball.
Which version of Photoshop do you use?
Which operating system do you use?
Please provide a copy of your 'System Info'. This can be get from the Photoshop menu item Help > System Info. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.
Try to reset the preferences of Photoshop using the steps described here: