99 Buick Century won't start

I have a 99 Buick Century. Inherited from my mother. Sat unused for approximately 2 years. Replaced the battery. Ran great for about a month. Was parked at night then died after starting in the morning. Was able to restart after a few minutes. Ran fine for a few more days then wouldn't start for a couple of days during a cold snap. Temp was tin the twenties for a couple of days. The temp warmed up to mid forties and now starts and runs fine. Have put gas in tank about 3 times since putting the car back on the road I think it was a frozen fuel line. I am going to change the fuel filter but would also like input on fuel additives and injector cleaners. To use or not? Thanks for your help.