Export settings have changed in LR?

As above but also do you have a Export Preset selected, Checked, in the Left most section of the export dialog?

If you do that is the problem. Once you select one you can't, in some cases, make any changes. Also it is best to post, included with the text of your post, a Screen Shot showing what you are seeing on your system.

Below is the proper way to Include a Screen Shot.

And the Export dialog with a User Preset checked. Note that everythiing is Greyed out but I can still expand or collapse different sections.

Some file formats (e.g. DNG) sizing, etc. However, most allow a wide range of custom settings. If yours are greyed out or not accessible, then it's possible you activated batch mode. An example of this can be seen in the green bounded checkbox on right side image of attached screenshot. To reactivate editable custom settings uncheck the box so that it looks like the left side image of attatched screeenshot.

Note that you're presets panel will likley look very different to mine. However, any presets you've created will have the checkbox that I've highlighted.

I've moved this from the poorly named Community Help forum (which is the forum for issues using the forums) to the Lightroom forum so that proper help can be offered.