Ctrl + Shift + L = Get a new Circuit (or click on the hamburger menu of Tor and click get a new circuit)
Onion websites have "circuits" which are temporary, so every now and then when you refresh the page you will get a new circuit which can be a slow one, or one that is saturated and can't access the site. This is why the "Get a new Tor Circuit" button is so prominent in the Tor hamburger menu.