On the picture you can see my Apple iPod Video 5th generation with the screen cracked. That happened when the original battery, which is located behind the screen, started to expand. It applied enough pressure on the screen to break it. So, if you know that your iPod needs a new battery, you should replace it sooner rather than putting it off. In my case I had already ordered a new battery and it sat on my desk for months. One morning I noticed the black spot and I thought that it might have been sun damage, but it could not have been! My laptop, my telephone, and my monitor are also on the same desk and get just as much direct sun on them. Then I remembered where the battery is in iPod and realized what had happened.
Now I had to replace the battery! Except, I also had to spend more money and buy a new screen.
You can find guides on replacing parts in your Apple iPod in iFixIt.com. Below I have linked to the ones for the iPod 5th generation. On iFixIt you can also buy parts, but if you have a device model number, it would be easier to search by model number than using the product filters in the left sidebar.