this is actually a serious problem and there are well 2 causes one is a hd sensor, the 2nd is the hard drive has actually lost its place for better terms.
The first fix is is simple and takes alot of time. But basically what happens is the lens on the inside of the hard drive skipped a beat from either being dropped or unplugged in the middle of transfering. If you were to take it into apple they wouldn't fix it under warranty or take it in return cause they consider it from being dropped. An it will cost you more then the units worth to get it repaired from most places..
first you can basically let the battery die. Let it sit with the red x until you cant get it to turn on anymore. You have to essentially discharge the battery. Leave it discharged overnight. Once the battery is fully discharged you need to allow it to fully charge. Once it fully charges it may come back and start working this only works on about 30% of iPod classic units. In some cases i have seen people just able to plug it in and let it charge for 4-6 hours and it will start working. but if, when you try to charge the iPod it shuts off after 5 minutes, your gonna have to let the iPod discharge.
Ok so now if that didnt work and you have discharged the ipod and recharged it there is only one fix left that many people have done and swear by. I myself had to use this on 100s of ipod classics and it works.
Reset your ipod by holding the middle and menu button until you hear clicking or immediately when you see the apple icon. then with one hand hold the ipod firmly and hit the side of it onto the palm of your other hand. I mean you really have to smack the iPod as soon as you hear that clicking or you see the apple icon. Sometimes this jumps the needle back into place but not all the time and may take a few times to try. Some times it takes 50 smacks to get it to work but it does work. I have known people who couldn't smack it hard enough so theyd hit it on a counter or table top, but dont do that the first 10 try's. Use this only as a last resort to the discharge/recharge fix.
You-Tube video "-gOWKhWyrOQ&feature=related" placeholder (do not delete)
heres a video that outlines the same thing in words. But doesnt show it being done.
Now if it took smacking your ipod to work when it comes back it should have all of your music. But in 10% of the cases it may not have all of the data left on it and it will ask you to repair or reformat. You will most likely have to reformat and lose your contents so i hope you have your library backed up but as i said thats only 10% of the time.
If you need further assistance please feel free to follow up with a comment and i will be here to assist you. If this assisted you please feel free to rate the solution. thanks and have a great day.