I just thought i would bring this to the community as this happened to me this morning and i have just had a phone call with a really lovely guy at Monzo who has cleared everything up at first.
I got a in app message saying my cards frozen and i need to order a new one due to suspicious activity. At first my defence gates flew up (I had a big issue with this 2 years ago where my account was frozen 4 times in the space of 3 months). Monzo admitted this was a system issue and said it will be fixed and this was put in the past.
I immidiately replied but got no response, so i phoned up after the second call got told that today there was lots of suspicious active card checks on loads of accounts and Monzo have froze affected accounts to protect customers. They have passed on to the premium team to order me a new card as i have premium and would have to pay in the app if i replaced the card myself.
Just a heads up if this happens to you