Locked out of iPad2 - Don't Remember Old Apple ID Information

10 years ago I received an iPad2 as a reward from the company I worked for. I no longer work there, but the iPad is mine to keep. It did not belong to the company. When I left the company I changed my Apple ID to match an email address that I control, because the Apple ID I used to set up the iPad was a company email address. I know my current Apple ID and password.

For some reason that I do not understand, today my iPad restarted itself and went into Application Lock mode. I am unable unlock it using my current Apple ID and Password, or with my old Apple ID and current password. I also am unable to contact Apple Support because there is no way for me get the serial number from within the iPad or from iTunes due to the lock.

How do I work around this? I did not steal the iPad. It is mine. It had no previous owner. When I received it, it was brand new.

Thank you.