A lie as a single act is one thing. A habit of lying or a lack of respect for truth are character flaws which are hard to live with. Lying out of weakness or shame is likely to be matter for therapy, but without therapy I hardly see a healthy relationship there. It depends what the reason was.
If she needs therapy, I’d insist she has it and the relationship shouldn’t move forward if she doesn’t. If she’s in a habit of lying, I would look for a different girlfriend. If you are so hurt by the lie that you won’t be able to trust her any more, I would consider taking a break if not calling it quits. I am assuming it’s she who lied, but the same would be true the other way round.