It's most likely listed under where it says you're muted, if you're viewing that you're muted from the appeal page. (Otherwise, where are you viewing it? ... If you're not viewing it there, go to Main Menu -> Account Recovery -> Appeal an Offence/Ban -> Login)
That path should bring you to a page like this:
Under the box that says this account is in the red zone, has 10 blackmarks, and is permanently banned, it shows the offence type (which happens to be offensive language), and you can view the evidence they have against you.
If you are at this page and there isn't any offences listed, possibly wait the 24 hours and check back to see if any evidence is posted. They may not reveal evidence when you're muted by a player moderator until it's reviewed by a Jagex moderator. (If it was a player moderator that muted you/if this is the process, I'm not sure.)
If no recent offence is listed and you wait a considerable amount of time and no offences are still listed, it definitely should be brought to Jagex's attention somehow. Posting on the official forums about it would probably be your first step since customer service queries have been removed.