Hello there Keijoie,
Thank you for using Apple Support Communities!
We understand that you are inquiring about how to unhide alerts for a certain sender in your Messages app. We are more than happy to give you some insight on this.
If you deleted that message thread, you would need to wait until the next time you receive a message from that specific sender, you would need to slide to the left on that message thread and press on "show alerts." For how-to steps on this, check out this article:
Use Messages on your iPhone, iPad, or iPod touch
Pay close attention to the section titled, "Turn on Hide alerts." You will follow those same instructions to turn off hide alerts.
We hope this helps! Have a great day!