Losing text styles

When I open drawings which I worked on a few weeks ago the text styles are wrong, and seem to have reverted to a simple font .  They seem to have lost the style I applied to them at the time they were drawn. The text style has changed from Ariel to Ariel.shx.  When I corrected it in Explorer the text changed to Ariel, but in Italics! When I press the Italic button in the Multilined Properties dialog box it reverts to standard - e.g. the Italic button seems to be inverted.  I am using version 7.1.0011 with Windows 7 64 bit. Could anyone offer an explanation?

David