How to unlock my Grundig SE 7270 TV?

I just bought a GRUNDIG SE 7270 DOLBY TV to my parents, and I was browsing around the menu, and I went into something like this: “Dealer’s menu”, and I’ve seen the opportunity to write there 4 digits, but after that the menu was frozen. I turned off the TV, also unplugged, but when I turn it on I just get a blue screen and it does not do anything for any button I press on the remote controller. It seems like I locked the TV, right? I’m so upset…. please help me, how to unlock it. Thanks in advance!