I have ringtones assigned to contacts and a special default ringtone. They suddenly do not work. Any thoughts?

If it's that your Custom Ringtones are no longer available/assigned to your Contacts/Default, then I had this same issue after the iOS 11.2.6 update -- for all of my Custom Ringtones had no longer even appeared as selections on the iPhone. I resolved this by removing all of my Custom Ringtones from my iPhone via iTunes and then adding them back again. Here are the steps that I took:

1. Ensure that your Custom Ringtones are in the Tones folder on your Computer, and leave that window open. I use a Mac, so that path is: [{Computer} -> Users -> {UserName} -> Music -> iTunes -> iTunes Media -> Tones].

2. Launch iTunes.

3. Connect iPhone to Computer (unlock iPhone, if applicable).

4. Select the little iPhone icon in the upper left.

5. Under “On My Device” select “Tones”.

6a. To remove one Ringtone, right-click on it and, from the pop-up window, select “Delete from Library”.

6b. To remove all Ringtones, select the first Ringtone, and while depressing the Shift Key, select the last Ringtone to highlight all Ringtones. Right-click on any one of the highlighted Ringtones and, from the pop-up window, select “Delete from Library”.

7. From the Tones folder window that you’d left open in Step 1, drag all (or selected) Ringtones over to the iTunes/Tones window.

Note A: I did not have to Sync in order for the iPhone to receive the Custom Ringtones; while I was adding them back, I could see them repopulating as Custom Ringtones on my iPhone: [Settings -> Sounds -> Ringtone].

Note B: All of my Contacts for whom I’d previously assigned Custom Ringtones (and that I hadn’t messed with since this issue had occurred), their respective Custom Ringtones were re-assigned properly; however, I did have to manually re-assign the Custom Ringtones to the few Contacts that I had messed with since this issue had occurred.

Hope this helps.

Cheers.