A Steam-layer problem with a Task Manager fix
If Counter-Strike 1.6 refuses to start, and particularly if it tells you only one instance of the game can run at a time, the usual cause is that a previous copy never actually closed. Steam sees a process it believes is the running game and declines to start a second one. Nothing is corrupted and nothing needs reinstalling; there is a leftover process to remove.
Play now — free, no downloadBrowse serversWorth separating, because the two get treated as one problem and they live on different layers. A fatal error means the game started, ran, hit something it could not continue past, and shut itself down. "Could not launch" means it never started at all — Steam declined before the engine got a turn.
That changes where to look. Engine-level crashes point at content, drivers and video modes. A launch refusal points at Steam's view of what is currently running, which is a much smaller and more tractable thing to check.
A report on Valve's own Half-Life issue tracker, number 2526, describes the mechanism precisely: after closing the game, Steam continues to believe it is being played, because hl.exe never exited. The reporter notes the game will not start again while that is true, and that another player had the same problem. The issue is closed, but the behaviour it describes is exactly what most people hit.
So the check is straightforward. Open Task Manager, look for hl.exe, and end it. Steam's status should stop claiming you are in-game within a few seconds, and the launch should work.
Restarting the computer does fix this, which is why it is the advice you find most often. It also takes several minutes and tells you nothing, and it will keep being the fix every time the process leaks unless you learn to spot the actual cause.
Ending one process takes ten seconds and teaches you what the message means. Reboot when that does not work, not before.
If no stray process exists and the game still refuses, the remaining Steam-layer causes are worth checking in order: verify the game files, confirm Steam itself is up to date, and check whether an overlay or an anti-virus is interfering with the launch. A launch blocked by security software often produces no message at all beyond the refusal.
If the game starts and then dies, you have moved to a different problem, and the fixes for a launch refusal will not apply to it.
The browser version needs no local process to be in the right state, which is occasionally the quickest way to establish that your account, your network and the servers are all fine and the fault is confined to one machine's install.
It is a way to keep playing rather than a repair. The leftover process is still there, and it will still be there next time until you end it.