Hi, every time I open a new drawing or click the AutoCAD icon, the undo buttons are grey out, I can turn it on at the command bar. But when closing autocad and open again, they are grey out again. I draw a line or something, delete it and no where to click undo due to button is grey out.Please help, thanks in advanced.

Solved:1. Open the CUI by typing CUI in the command bar.2. Find Quick Access Toolbars.3. Remove "Undo and Redo" commands from the Quick Access Toolbar.4. Added "Undo and Redo" commands back again in the Quick Access Toolbar.Done!I believe these two commands were corrupted along the way, somehow. 

You may have your UNDO system switched off - see the UNDO Control command.