A question I get quite often is why can I not double-click on my text or mtext and edit it or why can I not double-click on a block and edit it or something like that.
In AutoCAD there are a lot of variables that together make this work one way or the other or not at all.
Why does the setting change suddenly. It can be a program that changed the value and didn't restore it correctly or you might without knowing the consequences change the settings.
Here is what you should look for to get it working again.
The DBLCLKEDIT system variable must be on and the PICKFIRST system variable must be on (set to 1, the default) for double-clicking to work.
Tools menu>Options>Selection>Noun/verb selection changes the PICKFIRST system variable. Controls whether you select objects before (noun-verb selection) or after you issue a command.
Tools menu>Options>User Preferences>Double click editing selection changes the DBLCLKEDIT system variable.
Double click actions can be customized using the Customize User Interface (CUI) editor. In the CUI editor>Customize tab>click the plus sign (+) next to Double Click Actions to expand it. The double click actions are object type sensitive, allowing you to set up a specific command to use for a specific object type. Take a look there and see if this is the problem. The CUI file can be restored or reset if you have messed up with it and cannot recover the problem.
BLOCKEDITLOCK can affect if you are not allowed to double-click a block and edit it. When BLOCKEDITLOCK is set to 0 the Block Editor can be opened. When BLOCKEDITLOCK is set to 1, double-clicking a dynamic block in a drawing opens the Properties palette. If the dynamic block contains attributes, double-clicking the block reference opens the Enhanced Attribute Editor.
In some older versions of the Express Tools DCPROPS could interfere with the double-click behavior.