@@Ben799 You seem to have 5 listings, so I'm not sure which one this was to check how your listing reads. Do you state check-in times? If so, did you and these guests communicate about their arrival time beforehand, or did they just show up outside of your check-in window? It's important to get an approximate arrival time from guests and to tell them that if they are going to be late, to please let you know so their check-in will be smooth. I don't understand why the guests didn't just knock on the door, which probably would have woken you, when they didn't get an answer to their text.
I also don't understand why you just went to sleep when you knew you had guests arriving, without trying to communicate with them first to see when they were showing up.
In any case, there's not much you can do except give them a refund and offer a sincere apology. You could also offer them a free night's stay, if not for now, then for the future.
Don't beat yourself up too much over this- we've all had mortifying situations in our lives. One host who posts here often had almost the exact same thing happen with her first booking- the guests were going to be arriving late, so she told them she'd leave the front door unlocked for them and told them where to find their room. She went to bed. When she woke up in the morning, she realized, with horror, that she had, in fact, forgotten to leave the door unlocked, she had just unconsciously locked it as she normally does. She opened the front door to find the entire family sleeping on the front porch floor, covered in their coats. Luckily, they were nice people, she of course felt just awful, but they continued their booking for the next several days, and she had a lovely time with them and they all ended up laughing about it.