Hardware acceleration has been working since autocad LT 2014 came out. I recently got the cannot load heidi driver message and I cannot turn hardware acceleration on. Autocad tells me my ATI Graphics card is not certified but it has always worked fine until a few days ago
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Do you have windows automatic updates turned on? If so, try a windows restore to a " ...few days ago ..." and see if that fixes it.
Windows will also update your graphics driver if it sees it is a version behind. Sometimes newer is not always better.
PS: If you do have auto update turned on ... turn it off and update manually. That way when you restart you know right away that something is screwed and can remove the update (or do a system restore).