Z drops -3.79 Filament will not stick

[Mych]

Just bought a second hand Ender 3. The seller demo'd the printer with a very quick print. Basically a disc with a hole in it which he let run for a couple of layers about 20mins and then stopped the print. After getting it home I tried to do a print. Initially the test line was fine after about 1cm of printing the filament stuck to the bed and a nice line of even filament was laid. The head then moved towards the centre and started printing but the filament did not stick. I thought that maybe during transportation the bed was misaligned. I levelled the bed so that the head just pinched a piece of paper in all 4 corners about 3cm in from the edges. I went around several times. With the head in the centre of the table the paper was also pinched so I don't think the bed is warped. I homed the head and tried to print again and now within a few seconds of starting the head jerks and the screen show X 0 Y 0 Z -3.79. The menu button will not respond and all I can do is switch off at the mains. What have I done wrong? Thanks.