Thank you for you comment spyman. I just want to see the opinion of some more players.

what was your deadline ?

In 24 hour games, there is often a chance that something unexpected will happen that denies you access to a computer for the rest of the day. When you have a packed working schedule, and know that sometimes you might miss a turn, it is better to simply avoid 24 hour games.

any deadline above that should be easy enough to catch.

ATA, I respect players, like yourslef, that are fair. I strongly believe in fairness. However, there are a couple of reasons why it might be ok for him to have taken your supply center.

1. you assumed responsibility of every turn when you joined the game and failure to act is not the other players fault.

2. The other player doesnt know that you are not going to move before your turn is over, so maybe he just happened to have gotten lucky.
--- he has a plan and strategy of his own and why should he change it if you accidently missed a turn =/

3. (not a strong point, but here it is) You made all the players wait all those hours for you not to move, so at least one player should benefit from that wasted period of time. Waiting for that one single player to move without any reason is not fun for anyone that wants to keep the game moving.

Nonetheless, keep up your good and fair character. It will definitely help you in the long run. You may have lost one supply center, but you can make the most through continious diplomacy. Maybe make a fair exchange by supporting him into another SC and so forth. Cooperation is very helpful when you want to come to an agreement.

I think that it is perfectly fair that NMR is penalised by having all units hold. Just try to avoid it. Since we are all playing diplomacy, you shouldn't expect compassion.

Well, it was 12 hour game. The point is that I am usually on computer, so I play normally.
Even though I have not much free time, I check regularly the moves. However sometimes there are exceptional cases and you miss a turn.

So on the point, tell me if you consider my request "fair" or not. So far the score is 0-1.

your request is not appropriate, but doesnt mean you shouldn't try working something out. my opinion =)

Thank you for your comments. Indeed missing a turn is not polite and causes delay in game. Well, for me it is not very fun to win by an opponent's missed turn. The point is to play and enjoy, not just to win. (For me)

In the particular game, the moves were quite obvious, (quite stalemated) so everybody could guess what each player was about to play. Also there was one more player who missed a turn, so the waiting was not my fault.

Anyway, score 0-2 and one unclear.

Ok, score 0-3 then.

Sometimes I find myself in games where an alliance member doesn't put his order in on time, which is frustrating; but I don't think we can say to the other players "sorry we didn't quite get it together that turn. Would you mind if we let our team member put in the orders he was supposed to put through?" I realize that this is a little further down the slippery slope than your request lies, but the principle is the same, and I would be amazed if our opponents agreed. Still its diplomacy, and if you can get away with by all means try. Just don't be surprised when they say no.

仇～ATA～a similar thing happened to me on a poker table. I held 2 - 7s and someone raised - i folded but someone else called. The flop came over 7-7-3 I would have had a great hand. I told the other players this and they refused to hand me all the money. Can you believe that?...it's just not fair.

It is always fair TO ASK.
The issue is wether you think about the request being DENIED.
Trying to guilt a player for whatever reason is generally considered fair game.
Yielding to the guilt is another issue.
If the ONLY reason you could bring to your defense to get the center back then it is an issue of your inadequate approach.
What works best is that amongst a whole range of reasons for the return of the center there is also the issue of fairness so that the player can fixate on it maybe to put themselves in the best light without having to be seen to yield to your other issues.

So again, it is OK TO ASK, not OK to EXPECT ACCEPTANCE.

Nonsense. You snooze you lose.

Your ally relies on you. If you fail to enter moves that push the alliance forward, you are a detriment. There are new facts on the ground. i don't think he owes you that center back. Although you can refuse to work with him anymore. Within the new reality of his owning your center, does it still make sense for you two to ally?

It is a common event and I take the view that you must live with the consequences. Fog of War and all that: messages do get lost, misunderstood etc, and movements are affected. Even in face-to-face, players run out of time before submitting orders or are so harried for time that their orders are incomplete/incorrect... They are not allowed to make corrections after the event.

0 : Everyone

OK!
Thanks to everyone who have replied to this question.

The result is.
Your opponent did what every player in Diplomacy would have done. But asking for a center back is not really bad.