I’m fairly new to machine embroidery but I was going great guns on a stitch out when I had to change the bobbin and then suddenly the bobbin thread was all coming out on top. 
I’ve replaced the bobbin and rethreaded the top thread but nothing is working....

Any advice would be most welcome 

 

Is it a factory-bought bobbin, or who did you wind it up yourself?
Try to disassemble and clean the part where the spool is. Usually fine dust and thread scraps accumulate there. This causes the embroidery machine to malfunction. You can also check the bobbin thread tension.