There's this thing called corrosion. It doesn't always happen fast, but it happens after moisture is introduced to metals or electronics. Corrosion has caused this and may eventually kill the box entirely.
You should look at some of the questions and answers from the tag at right to see what you're in for.
At the very least you need to replace the power button (possibly the circuitry). Not many vendors have or will sell just a power button. The standard DIY repair for a liquid spill like this is to clean the logic board and replace the top case''. (An advanced DIY project).
If you decide to DIY start by studying the guide and notes for the project (not just looking them over). Obtain the recommended tools - every job is easier when you use the proper tools.
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