As always: Make a BACKUP first.
Which TW version do you use? … TWclassic or TW5 ?
The first thing I’d test is:
You can start with an empty.html and drag & drop import your broken HTML … If there is an $:/Import tiddler, you can click the “down arrows” in the table, to preview the content.
Import and see what happens. If you are lucky, the content will be there.
If the new wiki is also broken, try to start in safe mode. See link at the end.
The other first thing I’d do is:
Loading the broken wiki and hit F12, to open the browser dev console. The Console tab may show some logs. So there may be some hints what’s going on.
With the dev-console open I’d try. SHIFT - tab reload … to see what happens in the console.
Switching over to the Debugger tab. I’d activate - Pause on exceptions - in the Breakpoints setting. F5 - tab reload may stop somewhere, … This may be a hint
You can also look at: Tips & Tricks to salvage tiddlers from a corrupted single file TiddlyWiki
Have a closer look at: point 1 and point 4