Misplaced bishop rules

Years ago in a school tournament, in the middle of a game, we found that I had both bishops on the same coloured square. what are the rules concerning this? 

Capablancas rule:

You want your pawns on the same color squares as your opponents bishop, even if your bishop is on the same color.  

when starting a game, bishops stay on the same color square as their original starting square. one is on a dark color, and another is on a light color. they are constrained to that color Square. Obviously, if you promoted your Pawn to a bishop, you could possibly have two on the same color. but other than that, it's impossible. It is possible that you made a move and sidestepped your diagonal at one point in your game.

I made a move at some point, putting both bishops on the same colour. I did not have any bishops that were previously pawns. So it was the result of an invalid move. But it had gone unnoticed for too many moves to work out at what point the invalid move was made. My opponent stated that to abide by the rules, we had to declare the game invalid and start again. I never questioned him, but was he right? 

Starting the game over seems like it can't be right.  In official tournaments, notation is required so the mistake should be found.  At that point, you can either go from that point or perhaps play on from the current position

Although it was the school tournament. We played in our own time and there wasn't any notation. Now I think about it,  I wonder if he moved the piece in order to start again. There was motive as in he was losing. Thank you for the replies

In a real tournament, you use the notation to reset the board back to the last legal position.

In school tournaments it doesn't matter anyway. Just do whatever you want. Continue with both bishops on the same color, start over, give both players a win, doesn't matter.

In the USA, I was tournament director of many scholastic tournaments and problems like this occurred every once in a while.

I don't know if it matches FIDE rules, but the USCF rules say that if the last legal position was within the last 10 moves as determined by the players' scorecards, you have to go back to the last legal position and the Touch Move rule still applies, so the piece moved illegally must be legally moved if it is possible. The clock is NOT to be adjusted. Personally, I used the latitude provided to TD's and if there was less then 3 min. on either player's clock, I added an equal amount of time on each clock so both had 3 min. or more.  More time than that would potentially disrupt by too much the scheduled time of day for the next round.

If you can't find the last legal position because of mistakes in the score sheets or if more than 10 moves have been made since the illegal move, the game is to be allowed to continue in the current position - even with 2 B's on the same color!

@Snookslayer.  It blitz yes.  In long games, no

I think that's only for blitz.

It happened already to me several years ago – I placed a bishop on a wrong square from a different color.