There is no way for a responsive app like Roon to go and look into all of your FLAC files first before showing you for example the album browser window. So what such an app does during import, besides other things of course, is to extract the album art and store/cache them in some way for easier/speedier access. What the ransomware did, besides other things of course, was encrypting these cached pictures. That’s why Roon no longer can display them - they are no longer valid picture files.
What may help:
Have you tried to force a rescan of your audio files (from Settings > Storage)?
Have you tried to force a rescan from the album or tracks browser (select all then choose edit)?
If none of the above steps work to replace the corrupted cached album art, then I fear that only Roon Labs staff is able to tell you if there is something you can do about it at all.
Update - on second thaught:
If any of the above mentioned methods works, this will replace only the album art for local files - streaming content, artist pictures, radio station logos, genre pictures and so on may stay unaffected (still encrypted).