how many turns have you given truss rod adjuster? If just turning and turning is there any resistance? if not, it sounds like stripped threads - bad news
Plan A -
Slacken strings off, unscrew truss rod adjuster - does it extract? If so check threads on adjuster and rod. If you are lucky both are ok. If both ok, grasp bass neck with one hand at nut, one near body / neck join. Apply knee to middle of neck on fingerboard and push gently - you may hear creaks and 'cracks', do not worry. This should re-establish concave setting of neck and put truss ros at correct starting position. Fit truss adjuster back in, tighten until some resistance. String up, check action, loosen strings, tighten truss rod slightly, etc etc until all is ok.
If Plan A fails;
Plan B start as as plan A. Check if truss rod thread OK and the adjuster is stripped (in an ideal world this is what should happen, the adjuster should be softer metal than the rod), replace adjuster and move to plan A.
If Plan B fails;
Plan C - truss rod thread stripped - seek luthier advice / help or new neck needed, whichever is cheapest.
Hope you are lucky