I noticed on Wednesday a small crack appeared on the glass cabinet door in our kitchen. The crack feels like it's inside the glass not on the in or outside as both are smooth to touch, is there anyway to stop the crack spreading more? It already about 2 inches long, it was about 1 cm on Wednesday.

You can try cleaning the area with a solvent and then applying several layers of CA glue (super glue). However, there are no guarantees with glass and it will depend on what caused it initially (internal stress, accidental damage, heat/cold etc).

Your best bet is to get a glazier to make and fit you a new cabinet door, once the crack spreads from metric to imperial, that's usually the end of the story.

A new piece of glass shouldn't cost you too much, as long as it's not fancy.

Oh it's got a fancy design in it with lead and coloured glass, stained glass I guess. I recently moved the toaster over to that corner, I think the heat from it cracked it

see if you can find a stained glass repair place , they should be able to replace just the piece that is broken, without all the rest being affected. You could take the door in to them.
Call around.