You need to first reboot your system with Ubuntu 16.04 USB stick in live mode. Then open the disk utility app in your Ubuntu 16.04 and check if your hard disk is visible there or not (you will most probably found it there), if not then try to refresh a couple of time and see if your hard disk is listed. Also, try the following command :
This will list all the partitions on your hard disk.
From there select the required partition and try to format and repartition it into a single drive. Try the following command :
then in the command mode use 'p' to list all the partitions and then select the one which you want to format and format it using 'd' followed by the partition number in the command line.
After deleting create a new partition using 'n'.
(view the following link for a detailed process:
Once you find your hard disk listed, unmount it once and then again mount it and then start the Installation process normally in live mode.
If the above methods are not helping you then it might be a corrupted USB stick or Ubuntu image file, try rebooting the Ubuntu on the stick with "USB universal installer" or some other software and then try installing Ubuntu on your system OR try downloading the Ubuntu image once again.
Also, make sure you have created some unallocated space in your previously installed system to allocate that memory to the Ubuntu OS.
"GParted is a good partitioning tool which could be used for checking your hard disk and partitioning it."
I hope this helps you.