i have open the sealed box of iphone 7 and when i connected the charger it just show apple logo blinking and does not start

Try the steps in this article:

If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

If that does not help your phone might need service:

Contact Apple for support and service - Apple Support

Good luck!

Return it. No sense fighting with something that was supposedly new and doesn't work out of the box.

The iPhone 7 has been discontinued for almost a year.

Very old device and maybe the battery has died.