I encountered the same problem 2 days ago. Could not ping the DRAC IP either. So, I had to drive to the datacenter, power down 1850, unplug power cord, and plug the power cord back. Then, DRAC is back. I guess it must be a DRAC bug, maybe we should inform Dell to fix it.
The flash BOIS version on my 1850 is A02, which is the lastest according to Dell Support site. The DRAC 4 Firmware version is 1.20 (Build 03.11), which is also the lastest version.
I have 2 1850 with DRAC 4. And it's the first time I encountered the problem on one server. It seems quite random, but it's really annoying when it happens. Guess we need call Dell support and ask them to fix it.