when im trying to backup my phone to computer an error message is coming that phone is disconnected what i have to do?

Hello there, ashraq.

It sounds like you are unable to make a backup of your iPhone when syncing via iTunes. The following Knowledge Base article offer sup some great steps for troubleshooting:

If you can't make an iTunes backup of your iPhone, iPad, or iPod touch - Apple Support

After each step, try to back up your iPhone, iPad, or iPod touch again.

Restart your computer and your iOS device.

After restarting, try to make the backup again.

Unplug your other USB devices.

Unplug everything from your computer except your iOS device, keyboard, and mouse. Make sure you plug your iOS device directly into your computer.

Check your Windows security settings.

If iTunes for Windows stops responding or your backup never finishes, you might need to update, configure, disable, or uninstall your security software. Learn more if iTunes for Windows isn't responding.

Reset the Lockdown folder.

Try resetting the Lockdown folder to start or save a backup.

Try your secondary computer.

If you own more than one, try making a backup on your other Mac or PC.

Free up space.

If iTunes can't make a backup of your device because there’s not enough free space, try freeing some space on your Mac or PC. Then follow steps to make a backup again.

Get more help

Make a backup in iCloud or iTunes

Thanks for reaching out to Apple Support Communities.

Kind Regards.