This is a strange step to freeze on boot, may be you made an update but not finished correctly? Please make try these steps :
Once ready, open terminal and type the following commands
Try now to identify the partition where your Ubuntu is installed. Usually, the partition is named /dev/sdax, where x is the number of the partition. By example, my hard drive has
/dev/sda3 20923 23507 20755980 83 Linux
83 Linux that's mean the partition /dev/sda3 is formated under Linux (ext4). If you have several Linux partition, you will need probably try the steps bellow several time until you find the partition where Ubuntu is installed.
Now mount the partition like that
mount /dev/sda3 /mnt
Check if it's the right partition
If the file exist, you have probably mounted the right partition. Now we will try to fix the Ubuntu installed.
mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
apt-get -f install
If updates was not complete before, the process should be started again and finish the updates. If these steps doesn't fix your issue, check also if you don't have space issue with the following command when your linux partition stills mounted.
In last resort, you can try to place the few last line of the /mnt/var/log/syslog file content, this may help to identify where is the issue.