thing on power button has been lost

[Amitzack]

The thing on power button has been lost. what shoul i do. the flex cable is working fine , but the circular metal and waterproof seal has been lost.

[Phy]

Contact Sony Xperia support and see if you can get it replaced.
http://support.sonymobile.com/global-en/contactUs/

[Samuel1]

I'm sorry to hear that.
I suggest that you raise this with your local support team to proceed with this in the best possible way.
Local support.