You have found a problem with your condensing unit and you want to know if it is just normal wear and tear or if there is a serious issue that needs to be addressed. It could be a simple problem where the knob or switch has gotten a little worn over time, or it could be a more serious issue where the whole system has failed or been improperly installed. If you are unsure of what is wrong with your unit then you should check for damage to the housing that may indicate an issue. Usually the main housing is fine but any visible cracks in the housing will indicate that there is a problem somewhere else with the air conditioning unit. In most cases this can be repaired easily enough but you may need to replace the entire unit, which you may not have the budget for.
If you are able to find that there is a problem with the condensing unit, then the next step is to check the thermostat for any signs of failure. If you find that the thermostat is not functioning properly then you may have to replace the entire thermostat assembly. This is an expensive repair but it can be fairly simple if the thermostat is malfunctioning. If you have a fuse that is blown that may be the problem or it could be a more serious issue that requires the expertise of a professional repair technician.
The last way to tell if the condensing unit is not starting up is to look for any signs of dry rot. This is a very common problem with older units and if you are in doubt, or if you think that it may be a larger problem, it is always better to replace the entire unit. This is not only expensive but it can also be dangerous since you do not know what problems exist with a system that is not operating at full capacity. One of the main benefits of condensing units is that they are a lot more reliable than they used to be and that means that you get a lot of years out of your system before you need to replace it.