At the bottom of the main page click on about and you will see the admins, now click on one and send a pm.

In the footer there is 'About'. That page lists the admins. You can click on their name to view their profile and from there send a PM.

If you don't get a response to a pm, you could go to c/piefed_meta or c/piefed_help - most of the admins will be watching those communities.

Depends on which Piefed instance you're trying to contact and for what reason.
In the cases of feature requests a good place to start would be piefed.social/c/piefed_meta - !piefed_meta@piefed.social

If it's about contacting a specific piefed instance then check their about page, f.e.: https://piefed.europe.pub/about

If it's about helping out with developing etcetera then this page has links that will bring you to the right place: https://join.piefed.social/

So you can appeal a ban? Good luck.

you send them a pm