I accidentally ran "sudo umount -a" and now have no access to anything

Solution 1:

Just reboot the machine. It is just temporarily and the permanent configuration in /etc/fstab is restored upon reboot.

Solution 2:

Whenever you are on a situation like this on Linux, if you want to do a safe reboot, there is the "Magic SysRq key" + REISUB:

https://en.wikipedia.org/wiki/Magic_SysRq_key

You need to press: Alt + SysRq and while keeping that pressed type:

• r: The kernel takes raw control of the keyboard (from X or Wayland)
• e: Terminate all running programs
• i: Kill all programs left (except init)
• s: Sync (flush data to disk)
• u: unmount (remounts all filesystems readonly)
• b: reboot

Keep a few seconds between each typed letter for each of the tasks to complete.

This is a clean way of directly telling the linux kernel to restart.

Since you had unmounted all the file systems, the sync and unmount do not make much sense to me, however it is easier to remember the whole sequence than to apply only the parts you need.