First, are you getting any error/warning messages during POST ... specifically, are you seeing a memory/battery error? If so, DO NOT PROCEED - you need to do something different!
Second, you said "[all drives online], but that wasn't the case yesterday" ... this is not unimportant. What exactly happened?
Assuming the above items check out, then repairing in Recovery Console is your next step.
Make sure you are using this driver (assuming 32-bit) - use the Hard Drive version:
www.dell.com/.../poweredge-2850
When using nLite, at the appropriate time, make sure you select Multiple Drivers, select ALL that are listed, then choose TextMode. (When using an nLite CD, you do NOT use F6.)
If this still doesn't work, then you will need to check your PERC firmware version - if it is very old, then you may need an older driver.