iMessages won’t open.

“iMessage activation failed” pops up when I try to open the app. I’m already signed into my iPad so I’m not sure why I can’t automatically sign in to my iMessage, nor will it let me put in my username and password. I press cancel and the same error box pops up. I tried customer service, but it won’t take the serial number, even tho I copied it straight from the About section in settings.