My suggestion is to patiently ask lots of questions about what she did before the problem occurs. Also, if you can, sit with her for awhile, ask her to do again what she did the last time the problem happened, and observe what she does. By doing this, you might uncover the underlying problem.
Keep in mind that she doesn't process the details of using a computer as fast as younger computer users. This is due, to some extent, simply because she was not exposed to computers until much later in life than younger people.
My mother is a senior citizen. I'm not bragging but I truly do a better job of helping her with computer questions than some of the younger members of the family because I go at slower pace when explaining things to her. For example, one of my nephews (one of her grandchildren) might show her how to do something but he goes through it so fast that she simply does not retain anything he showed her.
