Well the boot drive would be your Hard Drive. Depending on what you computer lists the Hard drive may have lost connection or you just have to chose it. Can you list the full list? "etc ...." doesn't help give any exact answers.
Seems like the computer is unable to detect the hard drive as the primary boot device. You may try and restart the computer and follow the steps below to ensure that the hard drive is detected in the BIOS.
1. Restart the computer and tap F2 a few times on seeing the Dell logo.
2. In the System BIOS, under the boot device look for the hard drive.
3. It should show the size and make of the hard drive next to where the hard drive is listed.
4. We also need to ensure that the hard drive should be the first boot device in the boot sequence.
If the hard drive is detected, we can proceed to run the diagnostics on the computer. For running the diagnostics, restart the computer and on the Dell logo, tap F12 and select the option for running diagnostics. If there are no error codes reported we can then be sure of no hardware failure.
Once we are sure that the hard drive has not failed, we can then proceed to work on the software issues if needed.