Not being funny but have checked the following?
1. Wifi is turned on on kindle?
2. Correct passwordvhas been entered?
3. Your wireless router is working ok?
I would do the following in this order.
1. Un plug router, leave ten minutes then plug back in.
2. In settings turn on wifi on kindle?
3. Search for router and make a new conection.
Hope that helps and fixes it, mine sometimes looses conection and in find thisnworks.