Car that I drive regularly has flat battery, bad alternator?

A car that I pretty regularly drive everyday has just had it’s battery run flat.

I had just got back from a 30 minute drive and 10 minutes later tried to turn it on and it just kept cranking like a flat battery.

I haven’t noticed any weaker engine starting but I guess I may have not noticed.

Because I drive regularly and haven’t noticed weak engine start recently could this mean the alternator has gone bad?

Edit
I have since tried charging the battery and had no luck and have realised thanks to a comment that that was irrelevant anyway.

Edit 2
The car is an 1998 Nissan Pulsar N15 Manual