when i pluged my ipod classic 5g to PC then it shows “iTunes has detected an iPod in recovery mode. you must restore this iPod before it can be used with iTunes”?then after restoring the PIOD when IPod restarted then Itunes again detect the same ERROR.

What version of Windows do you use? If it's XP, what you describe is one of the symptoms of this problem

http://support.apple.com/kb/ts1493

I don't think it affects later Windows, but you can still take a look.

And what version of Windows do you use???

It's possible that there is a hardware problem with the iPod's hard drive, so the Restore is not completing successfully.

If you are using XP, and you went through the procedure at the bottom of the previously linked document to change the iPod's assigned drive letter, and you also tried putting it into Disk Mode...

If you have not already, try shutting down your computer and disconnecting all USB devices, including hub. Start up with only standard keyboard and mouse connected. Just to be safe, put the iPod into Disk Mode again

http://support.apple.com/kb/HT1363

Run iTunes. Connect iPod to a direct USB port on computer. iTunes may prompt you about "recovery" and ask you to do a Restore. Try to do a Restore again.