Open Settings
Tap Messages
Turn off Filter Unknown Senders if enabled
Tap Blocked Contacts
Remove any blocked numbers you want to receive messages from
Open Phone
Tap Recents or Contacts
Tap the info icon next to the number
Tap Unblock this Caller if available
Open Settings
Tap Focus
Turn off any Focus mode that may be silencing messages
Check your carrier account or app for message blocking settings
Restart your iPhone
Ask the sender to resend the message