hi, my apple watch 2 shut down unexpected and then if i want to turn it on need to push both side button for 5-7 sec

Hi

If your Apple Watch is initially responsive, then the following steps may help (check after each):

If you have not yet done so, update your iPhone and Apple Watch to the latest software, starting with your iPhone:

• Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support
• Update the software on your Apple Watch - Apple Support

Restart both your iPhone and your Apple Watch, turning both off together, then restarting your iPhone first:

• Restart your iPhone, iPad, or iPod touch - Apple Support
• Restart your Apple Watch - Apple Support
• If the issue persists, next close all open apps on your iPhone and then restart both devices again.

Unpair and re-pair your Apple Watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app.
• Initially choose to Restore from Backup during set up (most data and settings will be restored, with some exceptions, such as Apple Pay cards, passcode).
• If the issue persists, consider instead unpairing and setting it up as a new watch.
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

Otherwise, follow the instructions here, including contacting Apple Support (mail-in service may be available, if required), making a Genius Bar reservation or visiting an Apple Authorised Service Provider if necessary:

If your Apple Watch won't charge or it won't turn on - Apple Support