You do not say but have you checked the battery? Have you checked the battery terminals cable connections and verified that they are clean and tight?
If you have a DMM (Digital MultiMeter - use the Voltmeter function, these are available at most hardware stores, some cost under $10), with the engine off, check that there is at least 11.5V-12VDC across the battery terminals.
If not the battery is faulty or flat. Have you tried charging it?
With the engine running there should be between 13.8V -14.5VDC across the terminals. If there is less than this the alternator (or regulator within the alternator) is faulty and needs to be checked as it is not charging the battery.