my apologies for the short delay in reverting to you. I had to step into a meeting.
My immediate concern is that by changing the lock, you potentially may have exacerbated the problem because it is a breach of the protection against eviction act to evict a tenant without a warrant. accordingly, you may have provided the tenant with a basis to claim civil damages against you if you do not have evidence that they have voluntarily surrendered their tenancy and left the property.
Accordingly, you may wish to liaise with the tenant to advise that having been unable to contact the tenant, you have gained access to the property in order to properly secure it as you become aware that criminal damage was being committed leaving the property insecure.
you can go on to say that you have evidence that the damages that have been caused were undertaken by the tenant themselves and you would ask them to explain the damage that has been caused.
You can go on to say (if they have not returned the keys to the property or message due to surrender the tenancy) that there tenancy is obviously still in effect and they can collect the keys from (address / from a mutually convenient time/place) and that they continue to be liable for rent.
You may need to consider raising a claim against them for the damage caused to the property in the county court which presumably can be offset against the existing order made against you