Why is testicle swollen and hard after inguinal hernia surgery?

Could be a hydrocele: I would speak to the surgeon and request a re evaluation asap

Several causes.: A swollen hard testicle after inguinal hernia repair could be normal swelling or could be a sign of injury to the testicle or its blood supply. Best to contact your surgeon and be certain that all is well.

Congestion of testes: The testicle blood vessels travel through the inguinal canal. It seems likely that these were altered at the time of surgery, perhaps ligated. The testicle seems congested and I suspect a vein was ligated. This will improve over time, as more veins will develop. A varicocele can develop.