If you have a legitimate key that isn't working, or if the activation window won't let you proceed:
If you can't find your old code, the easiest way to play without headaches is to acquire a version of the game that has the DRM removed:
When the power returned, the badges lay on the floor, their glossy surfaces cracked like thin ice. The children, bewildered but unharmed, were gathered in a circle, holding hands, as Mrs. Patel read them a gentle bedtime story—one without any code, any activation, just words spoken from a human heart.
If you bought it through a service like Steam (though it is rarely available there now), you can often find keys by right-clicking the game in your Library and selecting Manage > CD keys . Availability & Alternatives
Considering the game's poor quality and numerous issues, I wouldn't recommend spending any amount of money on it. For a game that's supposed to provide a fun and engaging experience, "Daycare Nightmare" is a complete waste of time and money.