Neither the House of Lords or the Monarch can actually stop Parliament passing a law. They can in some cases slow them down, but if Parliament really wants a law passed it will happen.
This action to prorogue was however later deemed unlawful by the Supreme Court on the 24th September 2019 (2). See recent changes to senior members, and subsequent rulings on matters of state importance by the Supreme Court for a look at what happens when they try to correct parliamentary actions by the ruling party. They have been singing from the governmental hymn sheet ever since.
Whither democracy? Whither justice?
1. https://labourheartlands.com/parliament-has-been-prorogued-a...
2. https://commonslibrary.parliament.uk/decision-of-the-supreme...