A bench grinder typically requires 120 volt power, and there are any number of issues that can necessitate the exploration of the wiring in the grinder. When the grinder fails to start, or doesn't operate consistently there may be a problem with the wiring. It is wise to do some research before exploring the wiring of the grinder, as opening the casing of the grinder can compromise the wiring even further. Generally, there are three wires that properly route power through the grinder: white, black, and green. The white wire is the positive, the black the negative, and the green wire is the ground wire. All three wires will be encased in a plastic sheath from the power cord, entering the grinder, then branching off and running throughout the machine. There are a variety of connections throughout the machine, and the wiring branches off in various directions. It is helpful, particularly if troubleshooting the wiring, to have the original manual for the grinder, as the manuals typically provide detailed wiring diagrams for your particular make and model. Frequently, if you no longer have the manual or you purchased the grinder second hand and it didn't come with one, you may be able to download a manual from the Internet or order one from the manufacturer.
The wiring can certainly be one reason that the grinder is failing to operate are is not operating properly. The first step is to check the power supply and make sure there are not problems there. Also inspect the power cord to make sure it has not been damaged or severed. In a shop setting, it is easy for the power cord to be damaged. If you suspect that the power cord may be the problem, replace the power cord with a like rated power cord, or see if you can order a replacement from the manufacturer. This alone may solve the issue, and should be the first line of defense before opening the casing of the grinder.
If this is not the issue though, it may be necessary to open the casing to further trouble shoot the problem. Make sure that you have a manual before you attempt this, since as noted above, opening the case may damage the electrical wiring on the machine. Once the casing is open, you will likely be able to see that the wiring makes several different connections throughout the machine. Inspect all of these to make sure that the connection is still intact. Due to the constant vibration, it is entirely possible that a connection may be broken. Repairing a broken connection may be a simple as restoring it with a wiring nut or a little bit of solder. One particular point that you may need to pay attention to is the power switch. Make sure everything there is intact. Do not be tempted to remove light bulbs, as these are critical to the proper routing of power.