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Do other devices connect to WiFi OK?
Have you tried a restart, or 'soft reset', of your Kindle? if not, do so - and if you need instructions, let me know, please.
Can your Kindle 'see' the WiFi router in question?
If not, make sure
A. that the router is On (if it already is, try turning the router Off for 10 minutes or longer. Then turn the router On again, and try connecting again after about 2 or more minutes.)
B. that the WiFi on the Kindle is turned On (or, same thing: that the Kindle can 'see' something in the way of WiFi stations, even if not your router.)