The problem described below has been resolved. What I am looking for is understanding as to why it happened, what was affected, and how might I safeguard against the problem happening again.
From one day to the next, my ipconfig went from this:
Wireless LAN adapter Wi-Fi:
Connection-specific DNS Suffix . : hsd1.ut.comcast.net
IPv6 Address. . . . . . . . . . . : 2601:681:77f:c059:a1bf:653d:6494:f619
Temporary IPv6 Address. . . . . . : 2601:681:77f:c059:5c7c:a1a4:1235:a8f0
Link-local IPv6 Address . . . . . : fe80::a1bf:653d:6494:f619%5
IPv4 Address. . . . . . . . . . . : 192.168.1.116
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : fe80::b675:eff:fe63:f4a4%5
Wireless LAN adapter Wi-Fi:
Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::89f9:299b:1f80:bb7b%9
Autoconfiguration IPv4 Address. . : 169.254.187.123
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
Notice a few things that are different:
-Connection-specific DNS suffix was wiped-out
-IPv4 address was assigned 169.254.x.x (which I laterlearned was a symptom of the DHCP server assigning a default IP????)
-Subnet mask changed from 255.255.255.0 to 255.255.0.0
Default Gateway was wiped-out / unassigned. Which meant, of course, that I could not see or get to my router.
I tried googling and applying the solutions that made sense (and even some that didn't) including:
-ipconfig release / flush / renew
-netsh commands (various)
-unplugging power to my router(s), both cable and wireless, then powering-them back on (zero help)
-deleting letting Windows reinstall the wireless NIC adapter driver
-tried to hard-code DNS settings (IP and Gateway settings) instead of letting them be automatically assigned. This never helped.
-What finally worked was resetting Windows. Fortunately I was able to keep my files. But I am having to re-install all of my applications,including Microsoft Office, Visual Studio, Visual Studio Code, SQL Server,and Adobe Web CS4. It's a pain.
I know next to nothing about networking, but after hours of research on this problem it seems to have to do with my DHCP Server failing to assign an IP address to my client (to my laptop)?
Does that sound right?
I am wondering what can be done to prevent this from happening again? Is it the result of corruption that is fixable via RegEdit, or the reinstatement of some obscure file somewhere that I can backup and replace?
Would a deeper knowledge of networking under Windows 10 help me? Recommendations?
It seems to me that if a re-install of Windows 10 fixed this, then it is an operating file issue, which means some sort of file corruption that the re-install FIXED. So - what was corrupted?
Thank you so much. No problems the past few days since I re-installed.