How do you re-enable my runescape account

If by re-enable you mean recover a locked account then you can try the following:

• Try recovering it as a stolen account or file an appeal.

• It could have been that someone else got the account and did something else to get it banned/locked. Note: If this is the case, you can only try to recover it - if that fails, there not much else you can do but restart.
  

• Try answering ALL the recovery questions, put in the passwords that you know, email address (if the account had one). Also list the type of Armour you had and also some people on your friends list. Put the date or the estimated date that the account was created and the service you use (Mozilla, Safari, Chrome etc.)

• Try filing an appeal, but do not put someone else was using your account because if you do, it will remain locked due to safety.

Chances are it was because Jagex knew that someone else had access to you account ( they can tell this by a change in IP Address), Of which they may have locked it for Safety, but if this person did something bad in-game at the time they (may) have had access to it, then chances are the account is now banned.