Soak some fluffless material in water (I used kitchen roll) and place it in the space between the rails. Set the print to max paper thickness and manually move the print heads over the damp material. (It can be a bit fiddly). Leave it overnight or if possible all day and keep the material wet or replace it a couple of times. When you finally remove it hopefully It has softened the hard dried ink in the jets and will then respond to a couple more head cleaning operations.
Doesn't always work I'm told but it has for me.
Disconnect the printer from the mains first.