A game that shows a black box, spins forever or never starts usually has one of a few causes. Try these in order.
Many games show a short ad before they start. If the box is dark for a few seconds, give it ten. The game comes after.
The games here load from their developers’ servers, not from this site. A school or work network can block those servers even when it allows this site. If every game fails but the site itself works, this is almost certainly why, and there is nothing to fix on your side.
Some blockers stop games from loading along with the ads. Try switching the extension off for this site and reloading.
3D games need the browser to use the computer’s graphics hardware. In Chrome, open Settings, then System, and check that “Use graphics acceleration when available” is on. Restart the browser after changing it.
If you want to check, typing chrome://gpu into the address bar shows whether WebGL is working.
Lots of open tabs can leave too little memory for a game. Close some, then reload the game.
Very old browsers cannot run modern games. Updating usually fixes it.
Games do occasionally break at the developer’s end. Use the contact form and pick “A game is broken”, and we will check it.