You can always come back

An online match is run by the server, not by your computer. Your game window is a window onto it. That is why a dropped connection, a crashed game, a restarted PC or a different computer does not end your match: your seat and your army stay where they are, and you look through a new window.

This page says what that means today and what is still coming. Both are marked.

How to come back

Start W4 again, on this computer or any other.
Sign in with your W4 ID (or, as a guest, use the same name on the same computer: see below).
Join the same server.

The server recognises you by your account, not by your connection. You get your seat back at once and a complete picture of the game as it is now: your units, your gold, what you can see. Then the normal stream continues. You do not need to remember anything and you do not "load a save".

If your old connection was still open (the game looked frozen but was not closed), it is closed for you: the newest connection always wins.

What happens while you are away

The match goes on. The server does not wait for you (unless its owner turned on pausing, see below). Your units keep doing what they were doing, and stand when they are done. Nobody can take your seat or give orders to your units.
The others are told, in two steps. For the first minute you show as reconnecting: your connection dropped and you are expected back. After that you show as away. Either way your seat is yours. When the online service reports them, the Games list and the Server status page show how many players hold a seat, how many are connected and how many are reconnecting.
Your seat is kept until the game ends. There is no time limit after the match has started. Closing the game is never the same as leaving. The only ways out are the leave button, or the game ending.
After the game, the players stay in a room for a few minutes to chat and vote for another match. If you lose your connection there, you can come back to it and hear the result again; people who had no seat in the match cannot join it.
If everybody is away for ten minutes, the game ends with no winner. That is the only time absence ends a game.
Before the match starts, while the players are still gathering, a seat is kept for ten minutes for someone who is away; after that it can go to someone else. In the lobby, a seat is free as soon as you leave it.
An order is never applied twice. If your game resends an order after a reconnect, the server recognises it by its number and ignores the copy.

Two options the owner of a server may turn on:

Pause on drop. The game waits while a player is reconnecting. Each seat may hold the game up for two minutes in all, so nobody can freeze a match for ever by dropping on purpose.
Abandoned notice. When a player has been away longer than the reconnect minute, the others are told so, and that player's units stand idle. Nothing plays for them: when they come back they find everything as they left it.

If the server restarts

The official server saves the whole match to disk every ten seconds of game time and once more when it is stopped on purpose. If it crashes or is restarted, it picks the match up from the newest save, at most about ten seconds of game time behind, and everybody joins again as described above. For a planned restart the server first stops letting strangers in and waits for the match to end (up to an hour), so that most restarts never touch a game.

The Rejoin card

If you hold a seat in an unfinished match on a dedicated server, your W4 ID page shows a Rejoin your game card with the map, how long ago it started and the server's address, and a button that opens the game on that server through the w4://join/... link (your computer needs W4 installed so the link opens it). If you have no such match, there is no card. The same information is what the engine's start screen will use for its own Rejoin button.

Guests

A guest has no account, so there is nothing for the server to recognise except a secret it gave your game when you joined. Your game keeps that secret while it is running. If you close the game completely and the secret is lost, the server cannot know it is you, and the name stays taken until the match ends. Make a W4 ID if you want to be sure to come back from any computer. It is free.

What is not here yet

These are planned, and the page will change when they exist:

The engine's own "Rejoin" button when you start W4. The website card above works today; the start screen of the engine does not ask for it yet, so inside the engine you join the server again by hand.
Your camera, your selection and your control groups. The server can store a small note from your game per seat and give it back when you return (the protocol has it); the game window does not use it yet, so today you come back with a fresh camera and no control groups.
A computer player that plays for you while you are away. Today your units stand where they are.
Watching your own match from the browser while you are away. Spectating exists on the server (w4://watch/...), not in the browser.

If it does not work

"That name is taken" as a guest: the secret was lost; use another name, or sign in with an account.
"The game is full": the match you left has ended or the seats were taken before it started.
The server says nothing: look at Server status. A restarted server resumes the match from its last save, so try again in a minute.

See also Why is my game laggy?, Online play and, for server owners, Host a dedicated server.