It was early in the game. I moved a pawn that put my King into check by my opponents Bishop. Neither one of us noticed the check. We played on for another move until the check was noticed. Do I lose the game because of unintentionally putting my King in check and not noticing it?
No, you don’t lose the game because of this.
First of all, a King left in check should be considered an illegal move.
The position immediately before the irregularity has to be reinstated. This means you have to retrace all the moves until you reach the position before the illegal move was made.
If the position can’t be determined the game shall continue from the last identifiable position prior to the irregularity.
Go back to a position where the king is not in check and move something else.
Article 7: Irregularities
If an irregularity occurs and the pieces have to be restored to a previous position, the arbiter shall use his best judgement to determine the times to be shown on the chessclock. This includes the right not to change the clock times. He shall also, if necessary, adjust the clock’s move-counter.
7.4a. If during a game it is found that an illegal move, including failing to meet the requirements of the promotion of a pawn or capturing the opponent’s king, has been completed, the position immediately before the irregularity shall be reinstated. If the position immediately before the irregularity cannot be determined the game shall continue from the last identifiable position prior to the irregularity. The clocks shall be adjusted according to Article 6.13. The Articles 4.3 and 4.6 apply to the move replacing the illegal move. The game shall then continue from this re-instated position.b. After the action taken under Article 7.4.a, for the first two illegal moves by a player the arbiter shall give two minutes extra time to his opponent in each instance; for a third illegal move by the same player, the arbiter shall declare the game lost by this player. However, the game is drawn if the position is such that the opponent cannot checkmate the player’s king by any possible series of legal moves.
Read the FIDE Laws of Chess