Hello Tais17,
It sounds like you offloaded the WhatsApp app and upon redownloading it, you are missing your data from that app.
Here are some resources that can help:
While offloading the app "frees up storage used by the app, but keeps its documents and data" as seen in How to check the storage on your iPhone, iPad, and iPod touch, if that data is stored on the iPhone would be up to the app developer. From WhatsApp, check out How to restore your chat history.
You can also try to restore a backup, Restore your iPhone, iPad, or iPod touch from a backup. However, when doing this, you may lose any data from after the date of the backup. Also, you may want to check with the app developer to see if their data is recoverable from a computer or iCloud backup Backup methods for iPhone, iPad, and iPod touch.
For further assistance, contact the developer here WhatsApp Help Center or by following these steps How to contact an app developer.
Hope this information helps.
Have a good day.