One of my colleagues mysteriously could not use the delete key in AutoCAD. It worked fine in other programmes.
Generally this is seems to be due to a crash that resets certain variables (for some strange reason).
In this case the PICKFIRST variable was reset to 0. When PICKFIRST is at 0 it only allows objects to be selected after the command is initiated. The delete key does not initiate the ERASE command so the nothing happens when the delete key is pressed and PICKFIRST is set to 0, appearing to be broken.
If you set PICKFIRST to 1 then objects can be selected and an command initiated after this selection. The delete key then works as you expect it to do.