How can I force Ubuntu to boot on a stuck boot menu?

he answer to this one can be found in the grub file /etc/grub.d/00_header

make_timeout ()
{
    cat << EOF
if [ "\${recordfail}" = 1 ]; then
  set timeout=-1
else
  set timeout=${2}
fi
EOF
}

Setting the timeout value to -1 will stop the count down. Change the value to a value > 0 i.e. set timeout=10

this section of the file would look like

make_timeout ()
{
    cat << EOF
if [ "\${recordfail}" = 1 ]; then
  set timeout=10
else
  set timeout=${2}
fi
EOF
}

Then run

sudo update-grub2