There are multiple reasons you might have trouble accessing a particular .onion site. Here are just a few of the most common:
You’re not using the Tor Browser
The site you’re trying to access is no longer active
You’re trying to visit a site with a short v2 address. Only sites with 56-character v3 addresses still work
The network you’re using blocks Tor traffic
Your system’s date, time, or timezone is incorrect
Your antivirus or firewall is preventing the Tor Browser from working properly