I have a server that had a failed drive and a drive in Predictive Failure. I replaced the failed drive and it was rebuilding.
root@linux:~ # hpacucli ctrl all show config
Smart Array E200 in Slot 3 (sn: PA6C9%%BFTWZMV)
array A (SAS, Unused Space: 0 MB)
logicaldrive 1 (410.1 GB, RAID 5, Recovering, 1.4% complete)
physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SAS, 146 GB, Predictive Failure)
physicaldrive 1I:1:2 (port 1I:box 1:bay 2, SAS, 146 GB, Rebuilding)
physicaldrive 1I:1:3 (port 1I:box 1:bay 3, SAS, 146 GB, OK)
physicaldrive 1I:1:4 (port 1I:box 1:bay 4, SAS, 146 GB, OK)
Now the LUN is stuck at the status Ready for Rebuild. Is there anything that I can do besides rebooting the server?
root@linux:~ # hpacucli ctrl all show config
Smart Array E200 in Slot 3 (sn: PA6C9%%BFTWZMV)
array A (SAS, Unused Space: 0 MB)
logicaldrive 1 (410.1 GB, RAID 5, Ready for Rebuild)
physicaldrive 1I:1:1 (port 1I:box 1:bay 1, SAS, 146 GB, Predictive Failure)
physicaldrive 1I:1:2 (port 1I:box 1:bay 2, SAS, 146 GB, Predictive Failure)
physicaldrive 1I:1:3 (port 1I:box 1:bay 3, SAS, 146 GB, OK)
physicaldrive 1I:1:4 (port 1I:box 1:bay 4, SAS, 146 GB, OK)
"Ready for Rebuild" in this context means that the rebuild cannot start. Looking at your array configuration utility output, it's because one of the other drives needed for parity is also failing. Basically, you have two failing drives in a RAID 5 array. This is a good reason not to use RAID with without having an online spare. In this case, your only permanent resolution is to pull the data off of the drive and restore.
You can try a reboot (or power cycle). Maybe that will jumpstart the process (it's worked once or twice for me in the past). If you have HP support, you can also pull an ACU diagnostic to see the specifics behind the stalled rebuild.