According to the MTA:

The service disruptions were caused by smoke in the 63rd street tunnel according to FDNY personnel at the Roosevelt Island F train station. According to the Roosevelt Island Operating Corp (RIOC):

MTA has advised that due to a smoke condition there is NO “F” train service in either direction. Please plan your travel accordingly and visit mta.info or call 511 for more information.

For electronic updates on F Train service, sign up for MTA Alerts at mymtaalerts.com

and :

Following a previous disruption, the MTA has advised that F Train service has resumed. Expect residual delays and plan your travel accordingly, visit mta.info or call 511 for more information.

For electronic updates on F Train service, sign up for MTA Alerts at mymtaalerts.com.

Service resumed with delays at about 4:30 PM.