I recorded with my favorite bass (1992-ish Gibson LPB-1) a couple weeks ago and it sounded great. Haven't played it since then. Today I took it out of the gig bag and started playing and - what's this? I can barely play the lower frets, or open strings, especially on the D and A strings. No sustain, all buzz. The note just goes PLONK, and dies very quickly. It sounds like the string is almost lying flat on some other frets. It doesn't improve much until about the 7th fret. Visually it's hard to see what's going on... the action does look very low up there, but I can't spot any curvature of the fingerboard by eye or anything. Not sure what I should be looking for.
Has anybody experienced something like this? What could cause it to begin all of a sudden in between last time I played and this time? What should I do to diagnose the problem further?