Texturize problems

I have just imported a blog into WordPress.
I find that all special chaacters including em dashes, quotes, etc are all apearing as question marks (?). I checked the database in PhpMyAdmin, which remains intact.

I tried to use the texturize removal plugin here: https://teyandei.net/notes/wptexturize_removal_hack
However, the formatting problem remains.

Correcting everything manually would be an uphill task since there are over 4,000 entries.

Any suggestions?

TIA.