problem with connecting keyboard/mouse

When my iMac goes on standby mode it loses connection with the mouse and keyboard, so when i go back on to it i can't put my password in as it can not find the keyboard/mouse. I then have to force shut it down and make it restart to make it work.

I never had any trouble before, but this seemed to have happened since updating to El capitan

Batteries have been changed.

Any one have any idea how to solve this?

many thanks 😀