Assuming you’re running the same jobs, then a sudden change suggests a hardware problem, rather than the gradual deterioration brought on by crud accumulating on mirrors / lenses.
Check for a loose mirror or focus lens: the lock rings must be snug.
If the machine has molybdenum mirrors (heavy metal slugs), then they’re easy to clean. If it has gold-plated silicon mirrors, then they’re easy to damage; check for any surface imperfections.
Similarly, the focus lens must be clean. You must dismantle the laser head to get to it, but make sure it is both clean and undamaged. Assemble with the convex side upward, remember the silicone washer, and snug the lock ring against the lens.
Test the beam for the proper resonance mode, known as TEM00. A previous discussion goes over the details (start here and read the next few posts):
If the tube produces a good TEM00 spot when it’s cutting well and a bad spot (ring / blotch / whatever) when it’s not cutting well, then the tube is going bad. It seems they can flicker between “working OK” and “yuch” without anything else changing.
However, if the machine stopped cutting well and did not recover after cleaning and the test spot looks bad, then the tube has definitely failed.
Report back with pictures of what you find so we can look over your shoulder.