What will happen if my knee gets infected after a knee reconstruction, will it ruin the repair?

Yes: Infections following any kind of surgery can cause significant pain and morbidities. The critical point is that the infection needs to be eliminated in order for successful outcome. Depending on the kind of reconstruction, hardware or ligament grafts which are non-living and can be a nidus for microbes, are used. Sometimes, these can be retained, but sometimes they need to need to be removed.

No: It depends on the organism (mrsa is increasingly common and very virulent), whether its superficial or deep, and what type of reconstruction it is (e.g. Acl reconstruction, fracture repair, or joint replacement). Infections are always concerning, but not always a disaster. The sooner it is recognized and treated, the better the prognosis.