BT keeps blocking my account "because someone tried to access it". They failed to access it, so why block it? It's a veritable pain to have to keep resetting my password for no apparent reason. It usually happens after using a VPN.
The VPN IP address may be on a blacklist and as such it may appear to be a possible hacker trying to gain access to your account and as such the BT automated system is securing your account and making you aware of it.
Try not using your VPN or if you need to use it try changing the IP address location that has been allocated to you.
I've just come back from a Rhine river cruise using my O2 mobile phone for BT Mail access. My account was blocked six times in 10 days, requiring me to change my password each time which was very confusing as well as irritating. I was using a secure VPN or O2's mobile internet.