You most likely popped the head fuse. If you work on the machine without unplugging it and waiting 15 minutes, there is residual charge so when you take the head cable out it shorts and blows the fuse. That or the cable was plugged in crooked and that caused the short.
The fuse is on the main board and labeled F2. Depending on how old your machine is, it could have a soldered fuse or a socketed one.
Here is a link to the fuse that you can solder or pop into a socket.