You can reinstall the boot manager FreeBSD comes with in one of three ways:
Running DOS, go into the tools/ directory of your FreeBSD distribution and look for bootinst.exe. You run it like so:
...\TOOLS> bootinst.exe boot.bin
and the boot manager will be reinstalled.
Boot the FreeBSD boot floppy again and go to the Custom installation menu item. Choose Partition. Select the drive which used to contain your boot manager (likely the first one) and when you come to the partition editor for it, as the very first thing (e.g. do not make any changes) select (W)rite. This will ask for confirmation, say yes, and when you get the Boot Manager selection prompt, be sure to select ``Boot Manager''. This will re-write the boot manager to disk. Now quit out of the installation menu and reboot off the hard disk as normal.
Boot the FreeBSD boot floppy (or CDROM) and choose the ``Fixit'' menu item. Select either the Fixit floppy or CDROM #2 (the ``live'' filesystem option) as appropriate and enter the fixit shell. Then execute the following command:
Fixit# fdisk -B -b /boot/boot0 bootdevice
substituting bootdevice for your real boot device such as ad0 (first IDE disk), ad4 (first IDE disk on auxiliary controller), da0 (first SCSI disk), etc.