This ARTICLE covers many of the reasons, that media can show up Offline, but as Titles are not discrete files, but are in the code of the Project file (PREL), if they are showing Offline, then something is wrong.
Now, did you Export those Titles, creating discrete files for them?
Read the article, I'm 100% sure I haven't moved any of the files. It never pops up and asks me where a file is located, and I can still play the file from the side panel. I can also drag it from there into the timeline and it works fine. It's just some of the existing clips in the timeline that are showing up offline.
As for exporting, if you mean have I exported like the video as a whole including the titles as in saving a copy of the video to my desktop, yes I have, multiple times. But I've done that before in both this project and others and it hasn't effected anything. And if you mean something else by "exporting", then no I don't think I've done that because I haven't done anything I don't normally do when working on a project.
I also opened another project I'd been working on, added some titles, closed it, then reopened it, and those titles and all of the media show up fine. So I'm thinking it's just this one project. And normally I would just start over, but I've put a lot of work into this particular project and really would rather not have to start over.
I'd also like to note I am no expert with computers so you might have to dumb things down for me a bit, sorry.