On

On[symbol::tag] switches on a message so that it can be printed (reverses Off).

Examples

Re-enable a message:

Off[General::spell1]
(* ... later ... *)
On[General::spell1]

Enable tracing:

On[f]
(* Trace evaluations of f *)

Enable multiple messages:

On[Power::infy, General::argx]

Please visit the official Wolfram Language Reference for more details.