The only way to resolve this is to go through the complaints procedure - as there is an Ombudsman to help you if the bank does not uphold your complaint. You have protection and banks do this if they are suspicious of a transaction which took place and is a safeguarding system they have. They should tell you why they have done this though - they may suspect some sort of fraudulent activity but if they are not answering you, you should make a Subject Access Request (SAR) to find out what information they have.
Here is a useful link for a SAR request:
You also have a right to expect a service (from the bank) carried out with reasonable care and skill. Furthermore, the account service should be “fit for purpose” and “as described”. If it isn’t then you can allege breach of contract. You can report them to the Financial Conduct Authority on 0800(###) ###-####
You should now make a formal complaint and if that does not resolve the matter ask them for a "deadlock letter", which is a letter giving their final response. The bank's complaints policy will be on their website if you take a look.
Once you have the deadlock letter the next step would be to escalate this to the Ombudsman - they will investigate and liaise with the bankr which would hopefully result in a reinstatement of the account and access restored. They can order the bank to make a financial award for inconvenience if they have acted poorly.
Once you have their final response, you can make the complaint to the Ombudsman here : www.financial-ombudsman.org.uk/consumers/how-to-complain
Or by email:***@******.***
Based on the circumstances, assuming the bank does not uphold your complaint, I am sure the Ombudsman will do. The Ombudsman will look at this case independently and will make a decision based on what happened. Their decision is binding on the bank.