What word or phrase means "a loss of what was on your mind"?

Lost my train of thought, which is kind of the same as lost track.

Also if it is due to stress or overload you would say you were (brain) fried.

The most common expression that describes the instance when you suddenly forget what you were about to say is

• your mind goes blank

I was just about to say something, but my mind's gone blank

Alternatively you could use

• lapse

I had an awful lapse of memory when I was asked to talk about X.

via @Noah

• slip your mind

I meant to tell her that Nick had phoned, but it completely slipped my mind

For rhetorical terminology I suggest aporia or aposiopesis. In conversational English, this is termed drawing a blank (link to M-W—see also OED s.v. draw, v. 52.b). For slang, this would perhaps qualify as a species of brain fart.

In the US, such a lapse is often referred to as a senior moment.