Fix Lightroom Lost Thumbnails

After a full data transfer to a new Windows PC, plus two external drives now attached, Lightroom 4.0 (with the 4.4 update) launches and my catalog opens—file names, ratings and all metadata are intact—yet every preview is an empty grey square. I dragged what I believed was my most recent .lrcat backup onto the Lightroom icon, so the catalog itself loads, but the program clearly no longer knows where the underlying photos live.

Here is what I do know:
• The images were originally on the computer’s internal drive.
• I’m not sure whether anything was relocated during the move, and I haven’t checked in Explorer to confirm if the originals are still where they used to be.

I need someone who understands older Lightroom catalog architecture to:
1. Diagnose why 1:1 and standard previews are missing.
2. Re-link or re-map the catalog to the correct folders or drives, or guide me through it.
3. Rebuild previews if necessary so thumbnails display normally again.

Preference: A complete turnkey solution is highly preferred as I am not technically inclined. Quality of work is prioritized over speed, and while my budget is flexible within $250–$750 USD, I would prefer solutions closer to the lower end of this range if possible.

A remote session (TeamViewer, AnyDesk or similar) is fine; written step-by-step instructions I can follow myself work too—as long as I end up with visible thumbnails in Lightroom 4.4.

Please let me know how you’d approach the fix and roughly how long you expect it to take.