Start job is running for hold until boot process finishes up 18.04

Had the same problem because the disks were full, I had to free up space If you think this might be the issue you can select the advanced boot in GRUB menu and use the terminal option to remove some of the files then restart and try login again.

To use the terminal option, in grub select advanced boot, then select recovery mode, and in the recovery menu select root.

In my case I got this message when my disk filled up with some Timeshift backup data. I found out it by running

du -a / | sort -n -r | head -n 20

Which is 1) calculates sizes for each and every folder on a disk 2) sort them in descending order 3) print the first 20 folders (means largest on a disk)

and then doing a rm -Rf on particular Timeshift folder. Not sure what would be your case but if it is a space matter - I recommend to remove couple of large folder/files and check if that helps.

Here is the complete process of resolving this issue:

1. On the booting window, select the advanced option for Ubuntu to enter into the recovery mode.
2. Once you are on the recovery mode, select the root option to enter into a terminal environment.
3. Press Enter, then change the directory to the following location: cd /home/user_name/Downloads.
4. Enter du -s * | sort -nr | head -n10 to list top 10 largest directories.
5. Glance at the short list and enter rm -r .\name_of_file_to_be_removed.
6. Ctrl+d to return to the recovery mode, then resume the normal boot.

I also faced this problem.this because of memory will be full in your system. Clear some memory for it you will solved with the problem. Key board will not word at this time. So please turn off your computer. Open advanced option for Ubuntu during start up and clean the system, and clear the swap using the below mentioned commands ( https://www.google.com/amp/s/www.tecmint.com/clear-ram-memory-cache-buffer-and-swap-space-on-linux/amp/ )

On power up, can you press ESC or DEL or whatever key sequence gets you into the boot manager? I upgraded a LattePanda running lubuntu 16.04 to 18.04, and it was looping through some failure sequence. When I pressed ESC and entered the boot manager, I found that the first item in the boot sequence was an entry for Android. I don't have Android installed on this device, so I don't know how or why that entry was there. I selected the override to boot ubuntu, which worked fine. Once in the ubuntu O/S, I entered the following, based on info I found in another post:

sudo modprobe efivars

followed by:

efibootmgr

which told me that the Android entry was #5, so I then entered:

sudo efibootmgr -b 5 -B

to remove that entry from the boot sequence so that the next time I powered off and back on, it went to ubuntu. Hope this helps.