Answer: Wilting can be a sign of over or underwatering, of transplant shock, of moving shock (as in outddors to indoors abruptly), or of exposure to cold (a freezing night) or some sort of spray damage. Certainly you have had the plant for a long time and know how to take care of it well, so something unusual must have happened.
Of all those, I would wonder, since you repotted it, if there is a chance that the potting mixes did not match well.
That could cause the roots to stay in the original mix and fail to reach out into the new -- the roots keep encircling and growing denser and denser within the original soil. As a result, the plant could become dried out even though you think you have been keeping the soil moist enough. Another possibility would be that the new soil holds moisture better than the original (or the new pot is substantially larger than the original) and so the roots are too wet, thus causing a rot type of situation.
You might want to investigate the root situation and repot again using a different soil and/or loosening the root ball if needed.
Also, inspect very carefully for signs of pests. Scale, whitefly and aphids occasionally do attack this plant and can be treated with insecticidal soap.
I hope this helps you trouble shoot. Good luck with your hibiscus!
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