First try just disconnecting and reconnecting the internal battery. Then try turning the laptop on as in the last step.

If it's still not working, disconnect both the internal and the CMOS battery and hit both marked buttons. Then reconnect both batteries.

(in this case the date will have reset and on the next startup the BIOS will complain about it. You can either set it manually in the bios or boot up the OS which takes care of setting date and time for you)