I fell asleep at 12 in the evening and didn't say happy birthday to my girlfriend. Now she's not answering her phone or returning messages. What should I do?

In fact, you should have gone to her directly, or rescheduled your trip and then done what you planned to do on her birthday. Although she ignored you, it doesn't mean that she didn't receive the text message. If she cared about you, she would definitely have a mobile phone. So you should ask her out first. If you don't follow the original birthday plan, it is estimated that there will be no chance.