For a while I've experienced the same problem as aakodadi described in this thread. However, I can't remember this problem being triggered by any change in hardware configuration, and unfortunately, the solution described in that thread doesn't seem to work for me at all, since I still had to reboot my computer maybe 8 times afterwards before I got it to boot.
My setup:
hale-bopp:[user]:~$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 3,6T 0 disk
├─sda1 8:1 0 512M 0 part /boot/efi
├─sda2 8:2 0 25,6G 0 part /
└─sda3 8:3 0 3,6T 0 part /home
sdb 8:16 0 1,8T 0 disk
└─sdb1 8:17 0 1,8T 0 part /run/media/user/sidecar
sr0 11:0 1 1024M 0 rom
hale-bopp:[user]:~$
And an excerpt of what I think seems wrong in /var/log/auth.log.
May 5 02:28:22 hale-bopp kernel: elogind[1203]: Power key pressed.
May 5 02:28:22 hale-bopp kernel: elogind[1203]: Powering Off...
May 5 02:28:22 hale-bopp kernel: elogind[1203]: System is powering down..
May 5 02:28:23 hale-bopp kernel: elogind[1203]: Received signal 15 [TERM]
May 5 02:29:13 hale-bopp elogind[1257]: elogind is already running as PID 1203
May 5 02:29:14 hale-bopp lightdm[1261]: pam_unix(lightdm-greeter:session): session opened for user lightdm(uid=981) by (uid=0)
May 5 02:29:14 hale-bopp kernel: elogind-daemon[1203]: New session c1 of user lightdm.
May 5 02:29:14 hale-bopp elogind[1285]: elogind is already running as PID 1203
May 5 02:29:15 hale-bopp elogind[1301]: elogind is already running as PID 1203
May 5 02:29:16 hale-bopp elogind[1313]: elogind is already running as PID 1203
May 5 02:29:17 hale-bopp elogind[1316]: elogind is already running as PID 1203
May 5 02:29:18 hale-bopp elogind[1319]: elogind is already running as PID 1203
May 5 02:29:19 hale-bopp elogind[1322]: elogind is already running as PID 1203
May 5 02:29:19 hale-bopp lightdm[1308]: gkr-pam: unable to locate daemon control file
May 5 02:29:19 hale-bopp lightdm[1308]: gkr-pam: stashed password to try later in open session
May 5 02:29:20 hale-bopp kernel: elogind-daemon[1203]: Removed session c1.
May 5 02:29:20 hale-bopp lightdm[1261]: pam_unix(lightdm-greeter:session): session closed for user lightdm
May 5 02:29:20 hale-bopp lightdm[1261]: pam_elogind(lightdm-greeter:session): Failed to release session: No session 'c1' known
May 5 02:29:20 hale-bopp lightdm[1308]: pam_unix(lightdm:session): session opened for user user(uid=1000) by (uid=0)
May 5 02:29:20 hale-bopp lightdm[1308]: pam_env(lightdm:session): deprecated reading of user environment enabled
May 5 02:29:20 hale-bopp kernel: elogind-daemon[1203]: New session 1 of user user.
May 5 02:29:20 hale-bopp lightdm[1308]: gkr-pam: gnome-keyring-daemon started properly and unlocked keyring
May 5 02:29:20 hale-bopp elogind[1340]: elogind is already running as PID 1203
May 5 02:29:21 hale-bopp elogind[1349]: elogind is already running as PID 1203
May 5 02:29:24 hale-bopp elogind[1402]: elogind is already running as PID 1203
May 5 02:29:24 hale-bopp polkitd[1388]: Loading rules from directory /etc/polkit-1/rules.d
May 5 02:29:24 hale-bopp polkitd[1388]: Loading rules from directory /usr/share/polkit-1/rules.d
May 5 02:29:24 hale-bopp polkitd[1388]: Finished loading, compiling and executing 4 rules
May 5 02:29:24 hale-bopp polkitd[1388]: Acquired the name org.freedesktop.PolicyKit1 on the system bus
May 5 02:29:25 hale-bopp elogind[1423]: elogind is already running as PID 1203
May 5 02:29:26 hale-bopp elogind[1427]: elogind is already running as PID 1203
May 5 02:29:27 hale-bopp elogind[1454]: elogind is already running as PID 1203
May 5 02:29:28 hale-bopp elogind[1475]: elogind is already running as PID 1203
May 5 02:29:29 hale-bopp polkitd[1388]: Registered Authentication Agent for unix-session:1 (system bus name :1.44 [/usr/lib/polkit-gnome/polkit-gnome-authentication-agent-1], object path /org/gnome/PolicyKit1/AuthenticationAgent, locale en_GB.UTF-8)
May 5 02:29:30 hale-bopp elogind[1502]: elogind is already running as PID 1203
May 5 02:29:31 hale-bopp elogind[1511]: elogind is already running as PID 1203
May 5 02:29:32 hale-bopp elogind[1537]: elogind is already running as PID 1203
May 5 02:29:33 hale-bopp elogind[1550]: elogind is already running as PID 1203
May 5 02:29:33 hale-bopp dbus-daemon[1173]: [system] Rejected send message, 3 matched rules; type="method_call", sender=":1.57" (uid=979 pid=1561 comm="/usr/lib/geoclue ") interface="org.freedesktop.DBus.Properties" member="GetAll" error name="(unset)" requested_reply="0" destination=":1.0" (uid=0 pid=1179 comm="connmand -n ")
May 5 02:29:34 hale-bopp elogind[1571]: elogind is already running as PID 1203
May 5 02:29:35 hale-bopp elogind[1585]: elogind is already running as PID 1203
May 5 02:29:36 hale-bopp elogind[1612]: elogind is already running as PID 1203
May 5 02:29:37 hale-bopp elogind[1635]: elogind is already running as PID 1203
May 5 02:29:38 hale-bopp elogind[1646]: elogind is already running as PID 1203