We have no real information but it appears that PubMed and PubMed Central are inaccessible at present.
The alternative is Europe PMC - which actually has some advantages.
I hope their servers have the capacity to cope with a massive increase in load.
Be patient. I am sure they will do their best to cope.
(Currently having a break so not regularly posting but this its being discussed across the internet and could be of great importance. Or just a temporary technical issue. Whichever it is, members need to know.)