A bit more info may help:
1. Assuming you're using a PC, what version of Windows are you using.
2. Which version of iOS is installed on your phone.
I had the same issue with my iPhone 7 on Windows 7 recently and found that after trying all the usual remedies, un-installing ALL of the programs iTunes installs and then reinstalling iTunes solved my problem. If you decide to go this route here are the instructions.
Use the Control Panel to un-install iTunes and all of its components; do so in the following order only:
1. iTunes
2. Apple Software Update
3. Apple Mobile Device Support
4. Bonjour
5. Apple Application Support 32-bit
6. Apple Application Support 64-bit
On some systems, iTunes may install both the 32 and 64 bit versions of Application Support. Be sure to un-install both versions. It is not necessary to restart your computer until after the last component has been uninstalled. Following the restart of your computer, re-install the latest version of iTunes and again reboot your computer and you should be back to normal. All the components listed above must be uninstalled in the order given otherwise you may experience unexpected issues.
Good luck and let us know if and how you solved your issue.