You have multiple ways to do this, here is one :
1) Add a Timer behavior on your character ()
2) Add a Boolean variable on your character named for example canTakeDamages and set it to true by default.
3) On your character damages logic (the conditions / events where he takes damages) add the boolean condition, so by default he can take damages.
4) On collision with the power up, set the boolean to false, and start a time "onShieldOff".
5) Create a new condition "On timer" with the tag "onShieldOff", and add an action for the character where you set back the boolean to his original value.
Maybe I missed something but you have the idea !
(You should post this to the "How do I... ?" forum