How do I restart an android emulator?

In Android Studio go to:

1. Tools > Android > AVD Manager
2. Select your emulator device
3. Click "Cold boot now" to restart, or "Wipe data" to factory reset

you can try

emulator @avd_name -no-snapshot-load

it will clean load the device it worked for me.

You asked how to restart the emulator from ADB. But the title is framed in a way that pops the question up when searching about restarting it from the UI as well.

One option could be (but it depends on which tooling you are using and may not be available for all emulators/environments) is to tap and hold the Power button and then a menu will pop up with a Restart option in it.

This was a long-standing default behavior, but since Android 12 / API 31 it tries to open the Google Assistant instead (even when there's no such thing on emulators without Google APIs or Google Play 😵). The old behavior can be reverted by going to Settings > System > Gestures > "Press and hold power button" to bring up the menu in the screenshot.