iMac data recovery - file icon with ? mark
The folder with the question mark means your iMac can't find a bootable operating system. You can try the following:


Press and hold the power button for a few seconds to power down your iMac
Press the power button once to turn your Mac back on and immediately press and hold the ⌘R keys on your keyboard to try starting your iMac from Recovery. Keep them pressed down until you see an Apple logo or globe.
If prompted, select a Wi-Fi network to connect to the Internet as part of startup.
After the Utilities window appears, choose Startup Disk from the Apple menu.
Select your startup disk
Click Restart.


Hope this helps. Let me know how you go.

NOTE
If you don't see your startup disk in the Startup Disk window at Step 5 above, try the following steps:


From the Utilities window, open Disk Utility 
Select your startup disk from the left-hand side of the window
Click on the First Aid tab.
Click on the Repair Disk button 
When it's finished quit Disk Utility. 
Choose Startup Disk from the Apple menu.
Select your startup disk
Click Restart.