R is a registered trademark, which means you have registered it with the proper authorities TM is a self claimed trademark, which means that you include it on your first use and you continue to claim it via using the TM. If you have to defend it in court you have to prove that you had it in use before the trademark infringement happened and that you continued to defend it via using the TM mark. If you cannot prove that, then you run the risk of loosing said trademark.

There is also SM which is a service mark, TM is generally used for goods or products where a service mark is generally for services offered.

So to answer your question, they would loose their trademark and it would be publicly available for anyone to use.

This has happened and almost happened to many companies when their product becomes hugely successful and people start using their name as the generic name for said item.

https://www.businessinsider.com/google-taser-xerox-brand-nam...

https://www.gerbenlaw.com/university/trademark-symbols/