missing operating system message

Key question...Are these 2 HDDs setup for RAID or AHCI?

I suspect you have a RAID setup. The default BIOS setting is AHCI which won't see the OS in a RAID setup. 

Be careful here! If you don't have a RAID setup and change BIOS from AHCI to RAID, you could make the system unbootable and you could lose all your files. If you have a RAID setup, changing BIOS to RAID should solve your problem. Only you can make the decision to change the setting, especially if it's RAID0.

II you have a RAID0 setup, that's very risky. In RAID0, neither HDD has a full copy of any file. If one RAID0 HDD fails, you lose EVERYTHING.  So you should be backing up personal files on external HDD constantly to avoid total disaster.

Or, consider undoing a RAID0 setup (may require reinstalling Windows and all apps) and use one HDD for OS and apps, and the other for storing personal files. Even without RAID0 or 1, you should still be backing up both drives regularly, especially if these are the original (and old) HDDs Dell installed in your XPS 8300.

Since you replaced the battery, BIOS may have reverted to its default settings so it may not be seeing your boot drive.

Does this PC have a pair of HDDs, configured for either RAID0 (improves HDD performance)  or RAID1 (automatic backup)?  Or does it only  have a single HDD?

Disconnect all USB devices except mouse and keyboard. See if it boots now.

Reboot and tap F2 to open BIOS setup. Navigate to the SATA0 info and see if it shows the correct info for your boot drive. Then go to the Boot sequence and make sure your boot drive is listed as 1st Boot Priority.

Also check the System Configuration setting in BIOS setup, which will be either RAID or AHCI. Do NOT change this setting, at least not yet.

If you make a change to 1st Boot Priority to put the boot drive first, be sure to save the change when you exit setup and see if it boots.

I have 2 HDDs and the same product number is listed on SATA0 (and SATA1) and 1st Boot Priority. My system configuration is set to AHCI.

I don't know if my previous response went through (PC issues). I only see a C drive and have a D drive that is my original system backup for restoration. Does that mean I have a RAID, or am I possibly not using the second drive at all?

I'm confused... How did you check this if you were getting "missing operating system'" error?

When you have two HDDs, either in RAID 0 or 1, you'd only see one C: drive listed in Windows because Windows treats them as a single drive, with one drive letter (C:).  So are you saying you actually have three physical HDDs installed? 

Did you create the backup on "D" drive and manually do backups of  C: or does the system do the backups automatically (without any additional backup software)? 

Open BIOS setup and see if a 3rd drive is listed for the SATA 2 port or for any other SATA port (don't count the optical drive, if you have one).

On the BIOS setup, I have SATA0 and SATA1, no SATA2. I could only write on my C drive. My D drive came with the desktop, I did not create it. I think it was labeled RECOVERY, like a laptop I have.

Under the BOOT window, there was a diagnostic, which said I had two drives. The diagnostic program ran with no problems. I just can get past the 'missing operating system' message.

If you could only write to C: that suggests it's a RAID setup.

Recovery is typically just a partition on a drive with a copy of Dell's factory OS installation which allows you reset the PC back to its original state by reinstalling Windows and removing all software and personal files you stored on the drive.

You may need to change BIOS to RAID to fix the error, but you'll need to be prepared to accept the outcome if you lose everything - Hopefully not!  I can't make this decision for you.

Post back and let us know what happens. Assuming everything is ok, back up your personal files on external media ASAP!