Adobe Photoshop error saving files due to writing access

Hello,
Adobe Photoshop CS 5 created this error. Is the first time when I have this kind of error and I don’t have any experience to manage this. On the Internet no useful solution was found so I ask an expert on this website if can help me. Why I have this error ? What caused this and how can I prevent it ?
Thank you !

Could not save “AP-1600×700-2.psd” because write access was not granted.
OK
