I had the same problem a while ago, and I found that for me it was a combination of things so I'll just list what I found, and you can see if any apply for you.
You don't say what plate solve interface you are using, but I will presume you primary interface is Pinpoint, and your blind fallover is Astrometry.NET.
- Make sure you have the right resolution (field of view) for your setup. Ensure also you have set in your equipment setup the right focal length of your scope the camera is on. Doing a blind solve manually by right-clicking on an image would give you this.
- Make sure you have good focus.
- Pinpoint seems to always be the slowest of all the plate solve options on my systems. Install Platesolve2 as you might find it's much faster (and better in my opinion.
- Short exposure, well defined images should work well.
When the same thing happened to me, that it just stopped working after being fine before, I didn't know exactly what was wrong, and so changed from Pinpoint to Platesolve2. Since changing I have never looked back.
You could try changing your primary solver to the Astrometry.NET just to see if that works first. It will take a while, but usually not too long.
I hope something here clicks and helps.