I would first make sure the general comments option is turned on by going to Settings → Discussion and seeing if Allow people to post comments on new articles is checked. Also, make sure the checkbox labeled Automatically close comments on articles older than ## days is unchecked. Otherwise comments will be turned off for your older posts.
Then you can turn comments back on for all of your previous posts by doing the following:
- Go to Posts → All Posts.
- Because you have a large number of posts, I would change the display option to show a lot of posts on the page so you won’t have to repeat the following instructions too many times. Click the Screen Options tab in the upper right corner of the screen, and change Number of items per page from 25 to something like 100 (you can always change it back later). Click the Apply button, and you will now see 100 posts listed on that page.
- So now we’re going to do a Bulk Edit, so you don’t have to edit each post individually. On the drop down that reads Bulk Actions, select Edit.
Then click the checkbox next to Title so that all of the posts listed on the page are selected. Click the Apply button.
- A Bulk Edit section will open up. On the dropdown labeled Comments, select Allow, then click the Update button.
- All of those posts should now have comments enabled. If you have more pages of posts, go to the next page and repeat until you’ve updated all of your other posts.
- When you’re done, you can go back and change the number of posts being displayed back to 25.