Before this happened, my system warned that there was only 400MB of hard drive space left. The system was extremely laggy.
I thought it may be due to Timeshift backups, so I tried to open that, but it kept saying it was in the middle of a backup, so to try in a few minutes. This is the point where I rebooted and now I am unable to access my system at all.
It looks like the backups are what's caused the instability, but I've no idea how to get back on to my desktop to fix it.
Have you tried pressing the Shift key rapidly after turning on to get the GRUB menu?
If not working and you are at the point of the flashing cursor, try Ctrl+ Alt+ F1 to get a tty terminal open. If you can, you shoul be able to login in terminal mode and then CD (change directory) to where your timeshift snapshots are kept. Check the contents and remove the ones in between first snapshot and last snapshot.