I downloaded books and after that my gobii show usb connected. What can I do to let it work again??

What does it say when you try to open them?

Per Amazon: If your book won't open, there may have been an error downloading the book.
To download the book again:

1. Connect to Wi-Fi.
2. From Home, tap My Library, and then tap the Downloaded tab.
3. Press and hold the title of the Kindle book, and then tap Delete This Book.
4. Tap the Menu icon, tap Settings, tap the Menu icon again, and then tap Restart.
5. After your Kindle restarts, tap the All tab, and then tap the title of the Kindle book to download it again.

Amazon has several suggestions other than the one I listed above. Here is a link to another suggestion:
https://www.amazon.com/gp/help/customer/display.html?nodeId=201460960

Book Won't Open

Your book won't open on your Kindle (5th Generation), or there's an error when you try to open a book.
Before you perform these steps, make sure that your Kindle is connected to a wireless network. To learn more, go to Connect to Wi-Fi.
There may have been an error with downloading the book. Remove the title from your device, restart your device, and then download the book again.

1. From Home, select the book or content to be deleted and press the left side of the 5-way controller.
2. Select Remove from Device.
3. From the Home screen, press the Menu button and select Settings.
4. Press the Menu button again and select Restart.
5. After your device restarts, navigate to your Archived Items, and download the book again

Where are you downloading from? Do you have a good file manager?

Two methods.
First: A computer is required. Connect the old Kindle to the computer it should show up as a new drive. Open that drive and find the books. Transfer them to the computer. Plug in new Kindle and do the same only transfer the transferred books from the computer onto the new Kindle.
Two: If you purchased the books from sources like Amazon, your books are saved on their cloud. Simply connect your new device and download all your books again.

There are only three ways to load books onto your Kindle: (1) download them from Amazon via an account (using Wifi or 3G connection), (2) email them to your Kindle the unique email address for your Kindle which is assigned to you by Amazon at the time of registration, or (3) download the ebook file from a computer using the USB cable.

To email books to your Kindle or to download them via USB cable you must know the location of the ebook (which is typically a .MOBI file) and you must be computer-savvy enough to be able to either attach the ebook file to the email or to copy it to the 'document' folder of the USB connected Kindle.

There are several fora on the net, discussing the battery replacement for the Gobii ereader. It looks as if there is no service for this reader. Some people even say you should trow it away and buy a new reader.
I think it should be possible to open the reader and order a battery, as soon as you can get it out. Just use google to fine a replacement battery.
You also could visit a phone repair shop and ask them what it cost to replace a battery, but first check what a new one will cost, because in a shop it could cost more then a new reader.

Success

Most tablets are glued together, some have plastic connectors around the edge.
I could not find anything related to "Gobii" but I did find "Kobo"
https://www.youtube.com/watch?v=2SmYg4q9ai4

This video should only be viewed for reference, your device may be different and require different tools or pecedures to take apart.

You likely have to authorize it again as it thinks you have downloaded it as another non-paying subscriber. Contact the outfit you ordered it from.

To transfer books from CD to your Kindle you will need to use a computer and connect your Kindle to that computer by USB then copy the books from the CD to the Kindle folder that you can find via Windows Explorer.

See the help information for Transferring files via USB on the Kindle Support web site for further details on doing this.

If you have the 3G model of Kindle then you can download books using 3G which is like you would use with a mobile phone.

If you don't have 3G then you will need to connect to a WiFi network in order to download books directly to the Kindle. See the Connecting With WiFi Guide for details of how to do so.

You can also transfer books directly from your PC to your Kindle using a USB lead - see the video guide on how to do this.