If a conversation with a girl on Snapchat disappeared from your chat without you deleting it, there are a few possible reasons for this:
Automatic Deletion (Disappearing Chats): Snapchat has a feature called "disappearing chats" or "clear chats" that automatically removes messages after they have been viewed and the chat has been closed. This feature can be enabled by the sender, or it might be part of the conversation settings.
Unsaved Messages: If you have unsaved messages in a chat, they will disappear when you leave the chat screen. Snapchat automatically deletes unsaved messages once you exit the conversation.
Message Retraction: The other person in the chat may have retracted the messages they sent. When someone retracts a message, it gets deleted from both sides of the conversation.
Account Issue: In rare cases, there might be a glitch or issue with your Snapchat account or the app itself, leading to the disappearance of the conversation.
Blocked or Removed: If the girl blocked or removed you as a friend, the conversation would disappear from your chat.
App Update or Data Issue: Sometimes, after updating the app or due to data synchronization issues, conversations may seem to disappear.
If you didn't intentionally delete the chat and none of the other reasons seem to apply, it's possible that there was a temporary glitch with the app. In such cases, you can try restarting the app or checking for updates to see if the conversation reappears.
It's important to remember that Snapchat is designed to prioritize privacy and ephemerality, so conversations may not always be permanently available. If you need to keep a record of an important conversation, consider taking screenshots or using other means to save the information outside of Snapchat.