We had a similar problem to what Austin reported here: 69712 which seems to have kept the IFO down for the last few hours. 

In the attached trend you can see that the beatnote dropped suddenly and then returned to normal strength a few times. 

I lowered the threshold for the beatnote strenth to -30dBm now, (had been reset to -10dBm after the beatnote recovered).  To get the PLL to lock this time I had to disable the automation (using the request on the ALS overview and on the PLL screen), then manually adjust the crystal temp until the beatnote was close to 40MHz.  It seemed as though the laser temperature had drifted to the wrong side of the PSL, which the automation can't recover from without help.