I converted a channel into a structure and now my entries are listed in reverse order.
I have a very large number of entries. Is there a way or a plugin to permanently sort them and store the order?
I don't know if there's a quick/easy way to do this within the admin area, but if you go to Entries, then click on your Structure, there's a little 4-dot symbol to the left of the Entry's title (make sure you have Structure selected in the drop-down to the right of the Search bar). You can click that 4-dot icon and move them around manually. Doing this will reorder the way entries appear on your site.
If you're just looking to reorder the entries within the admin area (and not reorder them on the front-end site), then click the drop-down to the right of the Search bar. You'll see options like Title and Post Date. Once you click one of those, you'll see headers for Title, Post Date, etc., that you can click on to sort entries. Again, this method won't affect how entries are displayed on your site; this only affects the admin area.