BOOTMGR is missing. Press Ctrl+Alt+Del to restart.
1.) Boot your server from the installation disk (DVD 1 of SBS 2011 media)
2.) Click Next and then click 'Repair your computer'
3.) Select the first radio button, and then click Next.
4.) Click 'Command Prompt'
5.) Type Diskpart and press Enter.
6.) Type List Volume and press Enter. You would see a list of all the volumes on your system.
7.) The System Reserved partition is mostly around 100 MB in size. In the below example its Voulme 1 and is represented by the drive letter D.
8.) Type Select Volume 1 and press Enter.
9.) Type Active and press Enter. You would see a message 'DiskPart marked the current partition as active'.
10.) Type List Volume and press Enter. You would see an asterisk (*) against the volume that has the current foucs, in this case, its System Reserved partition.
11.) To verify the System Partition is now marked as Active, type List Disk and press Enter.
12.) Typically Disk 0 would be where your Windows is installed. Type Select Disk 0 and press Enter. You would see a message 'Disk 0 is now the selected disk'.
13.) Type List Partition and press Enter.
14.) Type Select Partition 1 and press Enter. You would see a message 'Partition 1 is now the selected partition'.
15.) Type Detail Partition and press Enter. As you can see the below example, the System Partition (which is Partition 1 on Disk 0) is now marked as Active.
11.) Exit DiskPart and reboot the server normally.