Lattepanda alpha cannot show anything on the monitor

I have one lattepanda alpha 864S board, and the LP board cannot show anything on my monitor after I launched the board now. I think the board may be able to launch the Ubuntu system successfully because the blue LED is on and I can see the cooling fan working. The signal light on the keyboard and mouse also seems fine. With the same devices, including the original USB power supply and monitor, everything can work well several months ago, and I have not applied power on the board for several months until recent days. I hope to get your advice to see if I can fix this problem by myself?
I also noticed that the RTC battery on the board is dead, only 0.06V remained. I am not sure if it is the reason because it should only relate to the clock. I noticed some suggested performing a "hard reset (https://www.lattepanda.com/forum/topic/312903)" with the board, however, I cannot find the reset button on my LP alpha 864S board. I tried to press the power button only, but it did not work.
Looking forward to your reply!