Rebooting the server will not affect the rebuild, so if it is useless to you in its current state, then you don't have any other choice than to kill it. If you can't get in via iDRAC either, then the problem is not the drives, as it works independently from the drives/OS. If you had an additional bad drive(s) in the server, it could have caused a read failure/rebuild failure/array failure.  Power cycle it to see if you can get back in via iDRAC so you can check out the CTRL-R PERC utility for further info.
I replaced a failed drive in our R510 server running Windows Server 2012. The drive was inserted while I was remoted into the server via RDP. About 15 minutes later my RDP session stopped responding and I am not able to get in via iDRAC either. All of the hard drives appear to be working, the busy lights are flashing, and the new drive appears to be rebuilding (the top light is flashing green), but it has been running for approx 7 hrs now. They are 3 TB 7.2k drives. I'm not sure if I should let it go for longer or power cycle the server, because right now the server is useless to me. Any thoughts/ideas?  I've never had a server stop being responsive during a disk rebuild.
Server not responding after replacing failed drive