This is pretty much a new problem this year - I have had occasional times before where the internet has dropped out, but it's usually for just a few minutes, or had a clear cause (like last year when someone from another ISP managed to disconnect me).
What I've been seeing this year is, every so often, usually weeks apart, the internet just stops working entirely, at some point between 12.05am and 12.20am (i.e. just after midnight). Then it doesn't come back up until hours later - usually I don't find out when, but I was up late last night and it came back a little after 3am.
When this happens the phone line etc. will be completely fine, and the modem will show all three green lights, but the router will say it's getting no internet, usually with the unhelpful error "disconnected by user" (which user? which end even?).
I wouldn't be concerned, except these events seem to be getting closer together. I've had three in the last two months. One last night, one on the midnight between the 12th and 13th of October, and one in November, which I failed to note the date of (I think it was mid-November).
I'm sure Plusnet will tell me they can't see any disconnection, but they repeatedly couldn't see any disconnection when I'd been physically disconnected and was for over a week, so I can't say I put much stock in that.