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I was tracing back processes and created a new process of electricity mix, I did not save any changes, however when I tried to calculate the previous version I got an error saying that the matrix is singular, what could I have done that creates such an error? Especially considering that I did not save changes.

I can not evaluate any process of ecoinvent and I have tried with different methods (EF, IPCC).
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Hi, when you did not save anything then of course you will not get different results from before, but maybe you did some changes and saved them, before? 

A singular matrix (the message refers to the technosphere matrix) cannot be solved, simply speaking; you get this e.g. when you have rows or lines in the matrix that are linearly dependent from each other, so e.g. just a constant factor different. This happens e.g. if you have two lines that are completely 0. This is the inventory calculation, so using a different impact method does not help. 

You can check the matrix by enabling matrix image export in openLCA (go to preferences) and then you can, for a product system, export the matrices from the calculation. Check the A matrix.

Hope this helps, Andreas