If you do not renew your trademark, the trademark will be considered dead. It means that you will lose legal protection in cases of infringement, and anyone else will be able to use your brand name or logo.
You have six months after the expiration date to still renew your trademark. Even after this period, there are several factors that can help you to register it again with priority. It is possible to claim a trademark, and previous ownership can help you in the registration process.
If someone tries to register your dead trademark, the outcome depends on many factors, such as the length of abandonment or actual usage of the trademark. When the trademark is not used for three years, it is considered presumptively abandoned. Logically, the longer the trademark is abandoned, the higher the chance for successful registration by someone else.