Is it just the digital display? Or just the speedometer?
If just the speedometer, there is a magnetic sensor in the front wheel assembly that is easily broken when the front wheel is removed. Mechanics that are unfamiliar with the C50/Vol will break the magnet tabs off when either removing or reinstalling the front wheel. There are 4 tabs that must align with the wheel hub.
My first question would be, did you have a new front tire installed recently or remove the wheel for any reason?
If the display, as douglaura asked, did you troubleshoot the electrical connections?
I believe speedometer is on fuse #3, together with fuel pump. From experience, I would follow these steps to troubleshoot:
1) If your your pump primes (vzzzzu noise when you first turn the ignition key on and release the kill switch - red button on the right handlebar) it most is probably not a fuse and you can jump to step 3. If pump doesn't prime, go to step 2.
2) If fuel pump doesn't prime, check the kill switch to be release, and then check fuses. If all fuses are OK* (not just fuse #3), but headlight, marker lights, running lights don't turn on go to 2.1, otherwise go to step 3**.
Note : I don't remember if the bike has to be in neutral or not, or kick stand has to be lifted for the pump to prime when in gear. I don't think so, but worth a try. It is so automatic for me, that I don't even think about it when I get on the bike.
2.1) Check if the main fuse* under the seat (30 A) is OK.
3) Check the connector underneath the back of the speedometer unit. Remove three bolts, on top and lift the speedo unit. It could also be a case of not being plugged-in fully after tank removal. Check all the pins for green death (corrosion), bent pins, etc. if everything is in order, you would need to check power with a volt-meter. It could be a broken wire, broke connection, etc. I believe power is on R/BI wire - red with blue tracker coming into the speedo unit. Troubleshooting electrical connections can be a pain, so if that is the case, you might have to fix a broken wire.
4) If all of the above have been done, and speedo is not responding, I would disassemble the unit, and look for any burn marks, corrosion, etc. on the board and components. Sometimes it is frailly obvious that something gave out/burned on the board. This should get you to a point where you could decide to buy a new speedo unit.
* Note that fuses don't blow just because. If a fuse is blown, it is usually a symptom of a problem in circuit. You could check it by replacing blown fuse, and trying to replicate the condition when fuse blows. Could be broken/shorted wire, connector issue, or issue with components (e.g. a shorted switch) that causes a fuse to blow.
** The fuel pump connector is under the tank, so it might have worked itself loose (shouldn't usually happen if installed properly) or has not been plugged-in fully after tank was removed/lifted. I had that happen once and the pump wouldn't prime. It happened after I've removed the tank, and didn't plug-in the connector fully.
Please report back as to what the problem is, should you find it. It will help others in similar situation.
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