Three possibilities:
Reboot into UEFI settings
Assuming that your machine still boots, you could try and reboot your current OS into the UEFI settings. This is possible on Windows (but off-topic here). On GNU/Linux systems, try:
sudo systemctl reboot --firmware-setup
And then reset the fastboot option. Note that this does not work for all UEFI firmware, depending on the quality of the implementation.
Change the grub default
Another way to get out of this catch 22 would be altering the grub default. Run:
grep -e "^menuentry " /boot/grub/grub.cfg
and see if it has UEFI Firmware Settings. IF it does, count the menuentries starting from 0. Then edit the default that grub will boot:
sudo nano /etc/default/grub
(or use vi instead of nano) and set the GRUB_DEFAULT= to the value you counted. Run
sudo update-grub