1. Restore from the automatic backup
This is the most reliable way to recover deleted messages. Uninstall WhatsApp from your phone and reinstall it from Google Play or the App Store. When you set up your account again with your phone number, the app will automatically detect if there's a backup on Google Drive (Android) or iCloud (iPhone) and offer to restore it before you can start using your chats. Accept the restore and wait for it to finish without closing the app.
2. Check how often the backup is made
Before giving up on a message, check when the last backup was made. Go to WhatsApp Settings → Chats → Backup. There you'll see the date and time of the last backup and the frequency configured, which by default is usually daily, weekly or monthly depending on what was chosen at the time.
If the frequency is set to "never" or to a very wide interval, change it to daily to reduce the risk of losing important conversations in the future.
3. Look for local backups on Android
Besides the cloud backup, WhatsApp also saves local backups directly on the phone. Open a file manager and navigate to the folder Android/media/com.whatsapp/WhatsApp/Databases. There you'll find files with the .crypt extension dated by day, which correspond to backups of your chats saved on the device itself.
These local backups are especially useful if you didn't have the cloud backup enabled, although restoring them usually requires renaming the most recent file and reinstalling WhatsApp so it detects it.
4. Ask the other person to forward you the message or photo
If the conversation is still available on the other person's phone, this is the simplest and most reliable solution of all. Ask them to forward you the message, photo or video you deleted: it doesn't depend on backups or previous settings, and the result is instant.
5. Be careful with "message recovery" apps
It's common to find apps in the app stores that promise to recover deleted WhatsApp messages without needing a backup. In the vast majority of cases they don't really work, since WhatsApp encrypts its conversations and there's no magic method to access messages that were never saved anywhere.
What's more, many of these apps ask for excessive permissions or request personal data to "verify" your account. Be wary of any app that asks for your number, verification code, or full access to your files in exchange for recovering a chat.