Most of Fluke's scopes need a good battery even when powered from the ac adapter (such as also the 190 or 199 series). If the power adapter is used and the scope is turned on and it just beeps and nothing else (or the screen momentarily flashes and nothing else) or nothing at all happens, a known good battery should be installed as a first step and this will usually fix it.
A battery pack for the 123 cost about $135 US. If you get the $30 knock-off, you're on your own.