If your speedo doesn't give a reading at all, then check the following:
Check all the wiring at the rear of the gauge.
To access the speedo from the back, open up the bonnet to access the rear of the dashboard.
Check wires are all plugged in and in order. Use the codes on the wires, against the chart below to match their location on the dashboard. (You can also download this chart below).
Check the fuse box.
Open up bonnet to access the fuse box.
Check the fuses to see if any have blown. Replace.