I just acquired an old ThinkPad 770X.
I had the same problem booting it up.
I only had the AC Adapter plugged in.
Then I plugged in a mouse, then rebooted the laptop.
Then, I set the time/date, and it booted to Windows ME (it was upgraded from Win98 to ME by the original owner).
The CMOS (3volt Lithium battery) CR-2025 located on the bottom of the unit, where the memory is located, was dead. This caused me to have to reset the time/date each time I plugged the AC Adapter back in, because the CMOS battery keeps the time/date stored in a chip on the motherboard.
I replaced the CR-2025 battery, rebooted the laptop, reset the time/date, now it retains the time/date normally.
The battery that came with it was dead, so I ordered a new one for around $70 (included shipping).
The original battery will only last about 3 years, so this one was dead a long time ago.