Trying other lenses or bodies is the most obvious, but if not practical...
Take some live-view shots, very carefully manually focusing fully zoomed in. See if those are also soft, as this takes all the focusing out of the issue.
Take some at very high shutter speed, bright light, VR off - make sure it's not some kind of motion blur, vibration that wasn't there (or VR that stopped working), etc. If better add back in one by one to your normal shooting setup.
Perhaps on a tripod, live view focus manually on stars or other point source lights, and take some images with a short exposure, but enough that you get a very bright somewhat over-exposed point. Not too over-exposed. Is it very sharp when zoomed to pixels, or is there haze or flare, that might indicate a dirty light path. Some flare is normal if too over-exposed, but a hazy area around it is not (unless, well, it was hazy that night). Dust won't cause this generally, but a film, fog or other wide spread dirt might. Getting a lens objective rained on might leave some residue, or if not well sealed may have gotten moister inside which is not coming out.
I've never seen a sensor dirty that presented as haze or blur, but I guess it is possible -- I would think that would show up if you examined under a magnifier as you might to clean it.
Damage to the lens? Any recent jolts, drops or jars? Trying on other equipment is one of the only ways to easily tell.
Oh... very silly question perhaps but... you didn't accidentally change from Raw to JPG or something like that? I once managed that somehow by accident, and was very surprised. Short of noticing the "JPG" they just loaded in lightroom, popped up to edit, etc. Same/same. Not nearly as good of quality though.