I believe your Master Boot Record is screwed up as it does not show Windows 8. Windows 7 boot manager cannot boot Windows 8. Did you at one time have both working and when you booted you could select either operating system?
Your screen shot only shows Windows 7 not Windows 8. As you always install the oldest version of Windows 1st, a reinstall of Windows 8 should fix your problem. If you have Windows 8 CD/DVD or on USB then boot from it;
If from a repair disk > Choose Keyboard type > Troubleshoot > Advanced options > Automatic Repair if that doesn't work if you have a recovery disk then;
Keyboard > Troubleshoot > Advance Options > Command Prompt then type the following: (hitting <enter> after each line)
bootrec /fixmbr
bootrec /fixboot
bootrec /rebuildbcd (scans for other O/S to add)
Reboot
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