You were asking
here about another electrical issue. Are these related? Have you done anything with regard to that issue that suddenly caused this to pop up?
Starter motors require a tremendous amount of current. When stopped, the are pretty close to a short circuit. A fully charged, healthy battery will have no problem supplying the large initial current for the starter and maintaining a current delivery to keep it turning. However, a dodgy battery (one that is discharged, one that has high internal resistance and so on) will not be able to meet these demands. The result will be that the voltage at the terminals will drop giving the appearance that everything is dead. The same type of symptoms can happen with loose or corroded connections at the battery (+ and -) and on the chassis (-). Bad connections means high resistance means a large voltage drop when current demands are high.
Check you connections first. Clean and tighten as required. Turn the interior lights on and crank the engine (or try to...) If the lights go out completely, your battery is likely done. With the engine running, what is the voltage at the battery terminals? It should be around 14V, give or take. If it's lower, you may have an alternator issue.
If the car fails to start after an evening of sitting (dead battery symptoms), the possible causes are varied: bad battery, bad alternator, phantom draw (e.g. trunk light staying on) etc etc.
Given that you're not sure about electrical stuff, I suggest taking it to a specialist to have the battery and charging system checked out.