I tried to re-size the HDD of my new Dell inspiron 7520 laptop and now I can not recover my old OS or even install a new one since during my new installation there is no HDD is visible.

I took an image using acronis software on a flash disk but I still can get it working

A hard drive needs to be initialized before it will be recognized for an installation. If it were a secondary drive you could see it and initialize it from within Windows, but since it's the only drive, you will have to use a boot utility to initialize the drive. You would also need an Acronis boot disk to be able to see the backup on the flash drive. Try searching for a free boot disk like UBCD or similar. I hope your laptop has an optical drive. They're starting to go the way of the floppy.

Almost sounds like your MBR got hosed. If you have the Windows Disc, try booting to that, and then choosing repair Windows after it loads. Bring up a command prompt and type the following commands in order:

Bootrec.exe /fixmbr
Bootrec.exe /fixboot
Bootrec.exe /rebuildbcd

If you do not get any error messages when doing this, reboot the PC to the HDD and see if it works. Hope that helps.