What Happens If You Run Out of Time?
In timed chess, the clock is a second opponent. If your time reaches zero, you lose the game, but there are important exceptions. Here is exactly what happens.
The basic rule
If your clock runs out, you lose on time (called "flagging"). This applies regardless of the position on the board, even if you have a completely winning position. The only exceptions involve insufficient mating material.
The insufficient material exception
If your opponent's time runs out, but YOU don't have enough material to deliver checkmate by any legal sequence of moves, the game is declared a draw. Examples of insufficient material: king alone, king + bishop, king + knight. However, king + rook or king + pawn is sufficient, so the flag fall is a loss.
Time increments
Many time controls include an increment: additional seconds added after every move (e.g., 15+10 means 15 minutes base time plus 10 seconds added per move). With increment, you theoretically always have at least some time for each move.
Online vs OTB
Online, the server enforces time automatically. In over-the-board play, a player must claim the flag (point out that the opponent's time has expired). If neither player claims it, the game continues. See time management strategies.