Two things I’d check first:
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Does the site load in safe mode?
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Are there any errors in the console? To see those you need to right click the page, select inspect, and then switch to the “console” tab.
If the site works in safe mode, then it’s an issue with a theme or a plugin. If the site doesn’t work in safe mode something else has gone wrong with Discourse. Looking at errors in the console will start to give us an idea of what exactly is going on.