If you have the original Ubuntu Live installation media (USB or CD), you could test running from that media, rather than the HDD. If that works with USB devices, then you could save any essential data on your HDD and reinstall -- or better, using the Live media, download and try a newer LTS version, e.g. 16.04, which has the Unity GUI with which you are familiar, or 18.04, which has reverted to GNOME. Being newer, these versions may have features more compatible with your PC.
The nice thing about most Linux distros is that they can be reinstalled in less than 30 minutes... as opposed to many hours for other OS's.