Pump the lever a bunch and see if that doesn't help.  Probably air from the reservoir got into the line.  It may work it's way back to the top now that it's on the bike.  If that works, be sure to check the fluid level before riding.

Often when you take off the master cylinder, air can get into the line.
If you can, turn the bars so that the master cylinder is angles so that the high point in the system is at the point where the fluid goes down into the master from the reservoir. Just snap the brake lever a few times and the air will burp up out of the line.
Much faster than bleeding down.
If you can't get that kind of angle with the M/C attached to the bars, unbolt the M/C and burp it.