I'm surprised that you were given no notice of the impending closure of your accounts. Barclays have closed my current and savings accounts but I was given about three months notice that this was going to happen. I was informed by letter - to my Thai address - and also by a notification to my phone that there was a communication waiting for me on their online banking site, which was exactly the same letter as I received by post.
I was requested to reduce the balances to zero by 31 August and then to close the accounts. Fortunately, I also have an IoM International Account with them, so transferred the balances to that account which, allegedly, I am able to retain, albeit at a cost of 40 GBP per month!!
Trying to close the accounts was farcical - I spent a very long time on the phone, being passed around various departments. No-one seemed to know what to do. I succeeded in the end but it would probably have been easier to do nothing as they would have closed the accounts anyway.
As to the question of where your money is now, they will keep it until you can provide details of somewhere that it can be transferred to. Or, they will send you a cheque through the post - which you might actually receive but will likely be useless unless you have a Thai bank that will accept it.
Obviously, you need to talk to them - and good luck with that!