Hi David here, The Famous Forry.
The problem here may well be the grid at the base of your Hotpoint Mistral. Unplug appliance, remove front grid and vacuum with good strong vacuum using CREVICE tool. You need to get down on the floor and clear the clogged grid filter thouroughly. This may well get your appliance up and running again. failing this then you will need iether a re-gas or control module that realy requires a trained refrigeration engineer to complete. Good luck with the grid cleaning. Possible success rating here is 70%.