Map won't update, GarminExpress does not find the watch while showing it connected.

That was the first thing I checked. I have a (bought used) 5X that has very worn out/corroded  contacts in the back and of the 4 cables I have, only two will work. The 5X Plus however is pristine and the other USB functions work with any cable (I can upload music via USB with no issue) and the watch indicates Connected with the green dot in Garmin Express. It's only the map update that seemed to hang/fail).

As I indicated above, I eventually was able to complete the map update on the 5X Plus. When that was done, I did the same thing on the 5X and (with the right cable) it went without issue.

It is a connectivity issue but does not look like it's the cable.

It is puzzling and disappointing that Garmin Express is not able to tell what is going on and simply hangs.

Yes, my D2 Delta is very picky whilst my wife's 5S+ connects every time (and she never cleans it!). 

Maybe the Windows Event Viewer shows what's going on with the USB connection, (not too sure if that is useful, am a Linux user). Maybe if this problem starts to happen more often it is worth contacting Garmin support. I have had a watch with a USB connector that was broken internally in the watch (according to Garmin support) - never heard of that particular issue happening on this watch type though.

73 ;-)

That's a good idea. About Garmin support, I have contacted them via chat several times (not about this) and I have found them really helpful. First off, I have never waited more than a minute or two to be connected and then, the first person you talk to is usually able to give useful advice without having to transfer you to a bunch of people. So kudos to Garmin for having actual, friendly and useful people manning the support lines. I did call them about the 5X Plus which I had bought used, and I found out I had 5 months of the original warranty left. It was a really nice surprise. So if that problem comes back, either they'll fix it or they'll replace it.

Out of interest, have you tried a forced power-off cycle? (hold power button for 10+ seconds to force off, then hold it again for 2+ seconds to switch back on). If it is a firmware issue this might help....

No I had not as far as the map update was concerned. The maps eventually updated. However, a few days later, I found my credit card had disappeared from Garmin Pay and I was not able to entering back in. I kept getting an error message like "servers not available". Tried several times over several days. Eventually called Garmin support who advised to disconnect the watch from the phone's Bluetooth stack, reboot the watch and the phone and re-pair the watch to the phone. Then I was able to enter the card again. So something happened with the watch and since the reboot (3 days ago) everything has been just peachy.

Its funny, my life is rebuilding workstations, servers, any computer based system. Even when I follow the steps I have performed countless times, one out of five times the machine does something totally unexpected...... I can have an unattended push install running on 10 identical spec machines and 2 will glitch out.

The nature of electronics, or some form of weird planetary alignment! Ha, let the conspiracy theories start, hehe....

Glad its working 

I write software for a living. I perfectly understand where you are coming from

I know rebooting is unfortunately the goto answer for a lot of customer support guys. The fact that the watch now works does nothing to explain what happened in the first place and why, and the fact that the error message was obviously wrong. But since rebooting allowed the device to work again and eliminated any evidence of what happened, there is no incentive to actually fix what caused the problem in the first place. 

.... and I bet like me you are one that would be much more comfortable to know the root cause!