there is no system on my start up disk, what do i do

Try running the Apple Diagnostics.

Boot into Recovery Mode or Internet Recovery Mode and see if you can repair the filesystem using Disk Utility First Aid. If that doesn't work, try reinstalling macOS.

If you still have problems erase the physical drive and either reinstall macOS or restore from a backup.

Here is an Apple article regarding the folder with blinking question mark (aka No bootable OS).

https://support.apple.com/kb/HT204323

From what you are saying, it seems that an OS is not installed on your system. If your Mac is capable of internet recovery, you can hold Option+Command+R to boot from internet recovery. But if your Mac it isn't capable of internet recovery and it has a recovery partition, on startup, you can hold Command+R until you see a apple logo, more information on this can be found here if you need it: https://support.apple.com/en-us/HT201314. You can use this to re-install an OS onto your system. Hope this helps.