See http://cxc.harvard.edu/mta/SOT_reports/sot-shift.html 

Shift L5816. 
    Start at 175:13:00 GMT

We resumed science observations with obsid 17697 v404 Cyg i
at 2015:174:21:40:06.470.

About 2015:174:22:45 (1845 local) we lost power to the building (cause unknown)
and went on generators with no impact on operations.

Until .....

OCC evacuated due to fire alarm at ~ 1005 LT. OCC was on support with Chandra
via DSS-65 at the time. As all pass requirements were met, OC turned off
Chandra ~ 10 minutes early. OC, CC, and GOT DO were the only personnel on the
floor at the time and evacuated. Cambridge Fire Department investigated the
cause and provided the all clear to return. They were back on the 7th floor at
~1035 LT.

Finally ....
Building power was restored by 2015:175:00:00 and normal operations
resumed off-generators.