Tor Onion websites can go down for several reasons mostly due to the Tor network being unstable, but every now and then, it could be due to seizure, server issues or address change.
Tor Network Stability
The Tor network is ran by donations and volunteers, sometimes the network is over loaded meaning connections and packets can be lost. In this situation you can try clicking the “new circuit for this site” button until the site connects.
Onion Site Seizure
It is very rare for a Tor hidden service to be seized but it does happen. The government can find exploits in sites to discover the true IP address of a Tor hidden service and seize the server. If this happens you may not be able to connect to the Onion site. They also might put up an information page to brag about how gay they are.
Sometimes the server hosting an Onion site may be down or over loaded. In this case you can’t do anything but wait.
Onion Address Change
Sometimes Onion addresses change, this can be for stopping DDoS attackes or if the site owner forgot his private key. If this is the case the only thing you can do is find the new link.