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It might be due to temporary software error. Try rebooting your kindle as follows:
If your kindle has a leather (or any) cover, remove it.
If plugged in for charging or connected to computer, unplug it.
Press and hold the power button for 20-30 seconds and then release. This should initiate kindle reboot that will take 2-3 minutes to complete.
Once computer reboots completely, check for normal operation.
Let me know if you have any problem (s).