My ISP email service can also be accessed as webmail; logged on from work and it came up with password being wrong (it was one letter and nine digits); tried again and again. "Forgotten password?" message. It came up with a couple of security questions and then let me change the password. Fine. Check email, go home. Looking back, I think I'd managed to transpose two digits, and then repeated the error.
Boot up Mac. Can't get online. Network status shows all OK up to ISP and internet. Spend best part of a day checking cables, phone line, Netgear settings, and anything else I can think of. Nowt, nothing, zilch, no connection.
Then, looking at Netgear settings, it slowly registered that changing the password for email had also changed the password for the Netgear to access the ISP. Change password in Netgear window to match new one and voila! it works.
Then had to change password in Mail so that it didn't keep asking me for it every time I checked for Mail.
No doubt over the next few days, I'll discover how many other connections relied on that password! I DO learn.... eventually!