unmet dependencies : linux-image-3.16.0-48-generic (55 and utopic)

The utopic keel series 3.16 is no longer supported. The original 14.04 LTS keel, the trusty series 3.13 has LTS (long time support) until April 2019.

I suggest that you try one of the following:

- to install the current keel of the 3.13 series (the one with the highest version number available) and reboot into that keel. If it works, you should be able to remove the 3.16 keels.

- to upgrade the hardware enablement stack to 14.04.4 (wily) or wait maybe 3 weeks for 14.04.5 LTS (xenial) according to this link (more risky, so back up your system before you start)

https://wiki.ubuntu.com/Keel/LTSEnablementStack