If you were translating flawlessly, and your mind suddenly went off, what should you do? starting over is surely an option, but any other solution?

Well, admit there was a slight mistake and start over.

It's not a bad idea to have some breaks between  translation sessions during the translation process in order to recover and come back more excited for the topic in hand.

In addition, i would recommend another solution for you if you felt mentally exhausted after translating for a while.. you can always have a small break and return to the text but before jumping to translation at once you can reread what you have translated so as to return to the same mood in which you translated the previous chunks of text

Unfortunately in such cases a translator has no other options but to start over!