You're asking us to solve a problem without knowing what you did.
Here are a few things to try.
Boot the system and start pressing F8, this should bring up a menu, select
"Last Known Good Configuration"
If that doesn't work, try the recovery console, assuming you
have a retail version or full OEM version of XP as opposed to the
manufacturer's recovery disks that don't have this option though they might
offer their own recover
scenario.
Be sure your system is set to boot from the CD. If you are not sure, when
the system first boots you usually see message about how to enter setup or
something similar and tells you to hit a key, sometimes it's del, sometimes
it's esc, just hit the key. This takes you to the system bios, you need to
navigate to where the boot sequence is set, put the CD-ROM drive first in
line.
Place the XP CD in the drive, save your settings and exit. The system will
reboot and should boot from the CD. If you see a message to hit any key in
order to boot from the CD, do so, otherwise, assuming your system supports
it, the system should boot from the CD on its own as it can't find
an OS on the hard drive.
XP Setup will begin by examining your system, don't worry, just let it run,
it's just copying some files to a temp folder. Ultimately, you'll be
brought to a menu. Choose, "Repair a Windows XP installation using the
Recovery Console, press R.
You will be asked for an administrators password. This is not any of the
accounts you've created for XP. It's a hidden system account for which
users are asked only to create a password during setup. Most leave this
blank. If you left it blank, when asked for a password, just leave blank
and press enter.
At the prompt type bootcfg /rebuild and press enter.
If that fails, try a repair install as follows:
NOTE, while a repair install should leave your data files intact, if
something goes wrong during the repair install, you may be forced to start
over and do a clean install of XP. If you don't have your data backed up,
you would lose your data should that eventuality occur.
Boot from the CD. If your system is set to be able to boot from the CD, it
should detect the disk and give a brief message, during the boot up, if you
wish to boot from the CD press any key.
Once you have pressed a key, setup should begin. You will see a reference
asking if you need to load special drivers and another notice that if you
wish to begin the ASR (Automatic Recovery Console) depress F2. Just let
setup run past all of that. It will continue to load files and drivers.
Then it will bring you to a screen. Eventually, you will come to a screen
with the option to (1) setup Windows or (2) Repair Windows Installation
using the Recovery console.
The first option, to setup Windows is the one you want and requires you to
press enter. When asked, press F8 to accept the end user agreement. Setup
will then search for previous versions of Windows. Upon finding your
version, it will ask if you wish to Repair your current installation or
install fresh. Press R, that will run a repair installation. From there
on, follow the screens.
