Hmm, another High Sierra "quirk"? Man, am I happy that I have not upgraded yet. Just too many strange issues to deal with (although one wonders if a lot of them are due to insufficient planning/preparation).
A couple of more questions:
1. What Mac OS did you upgrade from?
2. Did you just upgrade "in place", ie, have High Sierra overwrite what was there?
3. Do you have V10.13.1 of High Sierra?
4. A recent security arose with High Sierra, and Apple earlier today issued a Security Update. You might want to read the thread I started yesterday about this, along with a post I just made to that discussion about the security update:
https://www.mac-help.com/threads/high-sierra-security-issue.224098/
5. Are you using any third party (ie, non-Apple) applications? If so, did you insure that they are compatible with High Sierra? This link can help you with that:
https://roaringapps.com/apps
You might need to download and install some updates.
6. This one is
critical: are you making backups to an external device? If (hopefully) you are, what software are you using for that?
7. I guess you can also reset your modem and router (your ISP should have instructions on how to do that. Myself, I am on Comcast, and their procedure is to unplug the modem and router, wait about 20 seconds or so, plus the router back in, and then the modem).