UPCV Front door locks from outside but not inside. please can anyone help?

Hi, I am looking for any advice on this topic, I am completely at a loss.
My UVPC front door has a triple lock, and works fine from the outside. However, on the inside, I can close the door and it 'catches', but if I try to turn the key to lock the door, the key won't move. I actually can't remember if the door handle should be moved upwards for locking , but it doesn't or if the handle stays put for the key turning. (I have not lived in my house for some time, as it was rented out whilst I was away).
I would seriously appreciate any advice, even if it is just to get me started,

Sincerely, Julie