Was the 'I' gone (for a moment)? Can someone explain to me this phenomenon that has happened to me? 

I am not a serious Buddhist practitioner. I used to do mindfulness practice daily (20&ndash;30 min on 90% of days), just observing normal breaths or maybe counting the breath. 

Last night I didn't get proper sleep so I just started observing my thoughts. Images started appearing and disappearing and also voices (as if I were talking or someone else was). Whatever we call reasoning is again a sequence of images and voices. The 'I' existed in the images and in the voices (as in I did this, I have this work, I had headache..., something like that) and, at the same time, the 'I' did not exist and everything was just a sequence of thoughts (voice or image). As a consequence, any egoistical thing like a superiority complex or that kind of thing made no sense &ndash; but this whole experience lasted roughly for 3&ndash;5 mins. I am slightly hesitant to use word I this morning but yet somewhat comfortable. 

Can someone explain this phenomenon to me?
It happened that images are appearing and disappearing 


This is seeing.


  voices(may be a kind of I am talking or someone else) appearing and disappearing.


This is hearing.


  Can someone explain me this phenomenon?


All conditioned phenomenon that are subject to arising are also subject to cessation. You experienced Images that came and went, and voices that came and went. There's really nothing more to it than that. 

In Vipassana, the aim is to clearly see reality as it is. When we are hearing, we simply know it as hearing. If you heard voices in your head, that was still only hearing. It wasn't "your speech", or "someone else's speech", it was merely noise that came and went. To say anything more about it is to falsely ascribe meaning to reality, that reality simply does not admit of. The same can be said of whatever you saw, thought, smelled, felt, tasted.

Here is an instructional booklet on Vipassana practice you may find of some use.

The "I" is just a compounded formation of learned behaviors that is culturally and behaviorally conditioned as one grows, and helps one ( in some regards) relate to the world. I don't know why the "I" disappeared for you in this case, but what can be said is that now you can see that the idea of "I" or "Me" is impermanent just like anything else. We spend our entire lives building up the "I" into some idea that we like, but it is subject to ceasing like everything, and so is also not worth clinging to.