A dead battery is the most common reason a car won't start, and it is usually the easiest fix. The important part is working out why it went flat so you don't end up stranded again next week.
Jump-start from another car or a portable pack, connecting the leads in the right order and away from any moving parts. If the car starts and drives, keep it running for a good twenty minutes to put some charge back, then test the battery as soon as you can.
If the car dies again shortly after a jump, the alternator may not be charging the battery while you drive. If the battery is more than three to four years old and struggles in the cold, it's probably simply worn out. A parts shop or workshop can test both in minutes and tell you which one to replace.
Leaving an interior light on, short trips and long periods parked all drain a battery faster. A maintenance charger is a cheap fix for a car that sits unused for weeks.