I am not in the game right now, but you should be able to go into the edit HUD mode and right click the HP bar (R3 for console players) so that the name of the bar changes from blue to white. Then save the changes and it should be back.

If it is not blue, then there is a setting in the UI configuration under character settings to change.

If you do not see the bar, then somehow you sent it off your screen and need to possible reset the HUD to get it back.