I found a solution that worked for me. Hopefully it works for anyone reading.

My scenario:

I have a number Kasa Smart Plug Mini (they all look the same but turn out to have slighly different part numbers), 3 regular switches, and a dimmer switch. I purchased them all from Amazon in the last month.

I woke up one morning and my internet was down. My phone was connected to wifi but no data. I rebooted my network and everything I own began working again, except the kasa products. Not a single one reconnected! The best I could find on the internet was to reformat each unit and re-add them to my network. I wasn't even able to accomplish that, which was especially infuriating.

Solution:

I live in a neighborhood. A surprising number of my neighbors networks have reasonable signal strength in my house. I decided to do a "channel scan" for all 2.4 Ghz networks. It turned out that pretty much everyone, including myself was on the same channel. My router channel was set to "Auto." Not sure what the point of that is if it's not looking for empty channels. Anyway, I changed to an empty channel. Just like that, all of my Kasa hardware reconnected on its own!

In summary, I think there was too much signal traffic on the same channel. Every router will be different, but you can probably google how to change the channel for your specific router. If you have dual bands, its the 2.4 Ghz network you care about. Also they make apps for your smart phone that will tell you about the networks and channel usage near you.

Hope this helps anyone reading! 

I just had this same problem...loaded a firmware update on my router the other day, then half of my Kasa switches wouldn't connect. Tried several router reboots, several switch "restarts," plus turned off power at breakers to truly power the switches down and reboot. Nada....

So, I ended up brute-forcing it and doing something that I should NOT have had to do, but it worked. I went into my wireless router config and changed the DHCP address range to something completely different, which forced the router to drop all the IP reservations for my devices. Then I changed it back, and hit "restart" on all the switches that still wouldn't connect. That forced the switches to restart and grab a new IP. Everything seems happy now.

  @Armor LTC  I couldn't  re connect  the tplink