Is it a separate internal hard disk, (inside the case)? Is it a separate partition, eg the only hard disk is divided into 2 partitions, called C and D?
What happened to your backups? You said this D drive has important files, so I assume you have backed up those files you cannot afford to lose.
Tell us more about this "got restarted". What were you doing when this happened?
In cased this is an "Access Denied" issue, look at this article, "How to take ownership of files and folders in XP", and apply that to the whole D drive.
Yesterday my system (windows XP) got self restarted. after that when i looked in to the D drive the drive is almost empty with all my folders disappeared and showing only 1 folder and 2 files of total size 1.2 Gb. But when I saw the properties of D drive it is showing 10.3 Gb used space. The C drive is showing complete information what it was earlier. But system is not working properly. Its getting stucked after some time. Nothing works after that even mouse pointer also... what should I do?... some important files are there in system....