935 absolutely will not connect to my Android anymore...

I had the same problem. Did you by chance disable the location permission for the app in Android? Turning it back on fixed the issue for me.

Thanks for the prompt reply. I checked and location was indeed enabled for the app, but then you gave me an idea: uninstall/reinstall the app itself (granting it access to whatever) -- and that worked! A huge thanks.

Though, then I just had a major scare after logging into GC on my browser only to see a totally different dashboard and some 10 year old information. I signed out/back in and thankfully my 12+ years of activities are still there. Not entirely sure what happened there but thank god...

What version iOS on what iPhone?
Contact Garmin Support.

According the number of recent 1 star reviews (google play) for the Android version of GCM, a lot of people are seeing connection issues - many are running Android Nougat.

It is good to know that a reinstall can fix it though.

Installed the app on my wife's iPhone, same issue, contacted Garmin support.

Have you done a reset? That's the first thing Garmin are going to ask you to do.
Soft reset: Press and hold Power button until the device turns off. Turn on again.
Hard reset: Switch off. Press and hold Power, Start/Stop and Back buttons until the device beeps. Release Back button (keep the others pressed) and hold until beeps again. Release both and it should restart. I might have the button order wrong so if it doesn't work one way try the other(release stop/start first)...unless someone puts me right first. Be warned! You will lose all your settings, personal information and created activities.

One try would be re-installing the current firmware with Webupdater. This helped fixing connection issues sometimes.

But have you tried to use Garmin Connect Mobile or which app do you have? Because I faced the same problem and guys from the shop where I bought it, they are telling me that the problem is in the App or in my phone. And hard reset didn't help me (I don't know why)

I had this problem, with a OnePlus 3T running Android 8 Oreo which was previously sync'd to my 935, and after I removed it I couldn't get it to pair again. I did solve it, but not before spending an hour on the phone with Garmin support. The support agent spent most of the time telling me my obscure phone hadn't been tested by Garmin, and they couldn't guarantee it would work (even when I said it had been working fine for 3 months). He then offered to RMA the 935, so they could test it in their lab. Arrghh! So here was my solution, which I discovered by myself after giving up on Garmin support.

The instructions for pairing a 935 which was previously paired are terrible. I had to go into Bluetooth settings, and forget the 935. I also factory reset the 935, but in retrospect I don't think that was necessary (lost all my data, grrr). When trying to pair with the phone, it is important that the 935 display the 6 digit Bluetooth security code. If it doesn't, ensure you have forgotten it from the phone side, Bluetooth settings. If that still doesn't work, then perhaps you do indeed have to factory reset it. Then select Pair with Phone on the 935, and Start on the Garmin Connect app on the phone. Once the 935 is showing the 6 digit code, you have to leave the Garmin Connect app on the phone, and go to the Bluetooth security screen to type in the security code from the 935. And if you don't do this within about 15 seconds, the Garmin Connect attempt fails, and you're stuck again (until you forget the 935 in Bluetooth settings I think).

So, an absolutely terrible sequence, undocumented by Garmin, but which worked for me. Hopefully this will help someone else too.