Hi @paulstefan
How did you try to reset the BIOS (cmos)?
If you just removed the main battery and held down the laptop's Power button for 15-30 seconds then sometimes this is not enough. You need to disconnect the RTC coin cell battery from the motherboard as well. This is the battery that maintains the BIOS settings when the laptop is turned off and when this battery's voltage falls to <2.6V DC, sometimes the BIOS can become corrupted preventing a normal start or other problems with the BIOS.
If you did try this please ignore the above, if you didn't here's the procedure:
a). Disconnect the charger from the laptop if connected.
b). Disconnect the main battery from the motherboard. You don't have to remove the battery, just disconnect it.
c). Disconnect the RTC battery from the motherboard.
Here’s the [link|https://download.lenovo.com/consumer/mobiles_pub/v14_and_v15_hmm_201911.pdf|hardware maintenance manual] for the laptop. Go to p.55 to view the necessary pre-requisite steps and then the procedure to disconnect/remove the RTC battery. (shown as ''backup battery'' in the manual)
When the battery is removed measure its voltage. If it is <2.6V DC replace it. There are 3 compatible batteries for the model. The part numbers are 5B10S34067, 5B10S34066 and 5B10S34059. If you search online using any of the part numbers ''only'', you will find suppliers of the part.
d). ''Press and hold the laptop's Power button for a full 30 seconds and then release it.''
e.) Reconnect and install the RTC battery (or its replacement).
f). Reverse the above steps to re-assemble the laptop.
g.) When the laptop is completely re-assembled, connect the charger and try to turn on the laptop.
If it starts OK there may be a message about the date and time being incorrect. This is normal as the BIOS has been completely reset back to its default settings. Once the date and time have been adjusted the message won’t appear the next time that the laptop is started.
If you had previously changed any user defined settings in BIOS to suit whatever specific hardware configuration that you had for the laptop, then these will have to be set again.