Okay, I'll guess again:

Is there anything markedly different about the text file(s) you're
importing? Size? Content?

Can you test importing a file that worked before, using a copy of the
workbook?

Have you checked for possible viruses, spyware, and the like?

You could try booting in Safe Mode and running the macro. If it runs
as it used to, then something is loading on a normal boot that is
interfering with it. You can then try ending nonessential processes in
the Task List to see if one of them makes the problem disappear.