Unplug it (that’s important!), remove 2 screws under the lid at the back, slide the lid off backwards and you should be able to reach down inside the front and find the door release. Can’t remember the details but if you push the right bit of the lock it should open. Another possibility is a thin knife blade pushed in where the lock is.
Easy enough to replace the lock once you get it empty. I use http://www.ukwhitegoods.co.uk for spare and their forum for advice.
We’ve had Hotpoint machines for 25 years but the last one went to the scrapyard – one piece drum, meaning there’s no way to take it apart when the bearings go; won’t buy another. Yours may be before