According to the STM service updates website, service on the Orange line will be down until 9:15 AM.
The presence of smoke is the apparent cause, but there are few other details.
TL;DR: The STM Orange line is down because of the presence of smoke. Service is so far projected to resume at 9:15 AM. No details on the cause of the smoke.
UPDATE: It appears that the issue has gotten worse, or that the smoke has spread, because the STM has just tweeted that the closure as now extended past Snowdon to Lionel-Groulx station.
Shutdown ORANGE line between Côte-Vertu and L-Groulx. Presence of smoke. Service expected to resume at 8:59. #stminfo R
In addition to the extended closure, they are now expecting that service will not resume for another 20 minutes, at 9:15 AM.
Shutdown continues ORANGE line between Côte-Vertu and L-Groulx. Presence of smoke. Service expected to resume at 9:15. #stminfo K
Shutdown ORANGE line between Côte-Vertu and Snowdon. Presence of smoke. Service expected to resume at 8:59. #stminfo N
The STM tweeted the above notice to passengers at 8:32 AM, meaning they anticipate needing half an hour to right the issue.
The closure is currently affecting stations between Côte-Vertu and Snowdon.
No details on the cause of the smoke, or if there is an ongoing fire.
We will update you when we have more information, or if the shutdown extends past 9 AM.