Hi!
So I had an issue booting up my partner's new Versa 4 and I hoped to find the answer in the forum, but no joy.
However I was able to fix it myself, and I think sharing my solution may help others.
Firstly, if your watch only shows the fitbit logo and then switches off, you will find on the forum that you can hold the side button for 10ish seconds to 'hard reset' the watch. Some people may say this is a factory reset, but it's more like restarting your computer when something stops working. Some people my also say that you should reconnect it to your charger (I'll come back to this).
If reseting the watch doesn't work, hold the button again until the logo appears, then (quickly) click and hold it a second time. You will now see a different screen, and the logo will appear with a circle. Then you will see a loading bar. At his point leave it on charge, pop into your kitchen, make yourself a cup of tea, take a few deep breaths, then have a look at the watch again. Hopefully you should now see the language selection screen!
When the watch finally booted we saw that the battery was 98% full, so the issue wasn't due to the watch now having enough juice out the box. However, still try charging it first before factory reseting the watch, just to rule it out as a cause.
There were also a few issues when we paired the watch with my partners phone. The watch did connect, but the app and the watch froze (the green tick screen). We then had to hard reset the watch (hold ten second and let go) and close a reopen the app (we also switched bluetooth on and off again). Then we had to update the firmware on the watch. The app does guide you through firmware updates, and it's exactly the same has updates on your handset.
As a disclaimer, this method would only apply for watches out of the box. If you factory reset your watch, and you have already have data saved to the watch, it will be wiped (completely!).
I hope this helps!
