How to enter standby or hibernate mode in Server 2008 R2

Did you try the following in an administrator console:

powercfg.exe -h on

Also see MS KB920730

With the HyperV role installed you cannot standby/hibernate and there is no way around this (that I have found)

Also, I suggest Win2008Workstation.com for some resources as I assume you are using in a non-server environment

Just running up vanilla install to confirm about hibernate/standby for a server product from MS

Provided hibernation is enabled as described in the earlier answer (powercfg.exe -hibernate on), you can also hibernate from the command line using the command

shutdown /h

The the Shutdown command doesn't have a sleep option though.

I've seen various postings suggesting that one of the command's below can be used to trigger sleep / standby but on my system it triggers hibernate and I think the behaviour is dependent on your system settings. The documentation for SetSuspendState function describes the sleep capability but I suspect it has something to do with using rundll32 to call the function. The "Standby" or "0,1,0" parameter is being dealt with as single string and SetSuspendState was not written in a way to support the rundll32 interface specifically.

rundll32.exe powrprof.dll,SetSuspendState Standby
rundll32.exe powrprof.dll,SetSuspendState 0,0,1