2005 Monte Carlo (comment to Rolomatic's good answer) are the magic words! Your '05 Monte Carlo uses GM's infamous Passlock passive security system, and disconnecting the battery frequently makes it mad. You can soothe it with the 30 minute relearn procedure in the Common Problems section of the source. Be aware that all ten minute intervals are not a second less than ten minutes... some sources call it eleven minutes to be sure.

Have a free 15 second battery load test done at any store or new car dealership that sells new batteries other than Auto Zone. A battery load test will tell you the reserve amperage capacity. Amperage spins starter motors not voltage.

Are all four ends of your battery terminals clean dry and tight?

With no model / year info. If the battery is good, the factory alarm may need to relearn the key security chips.

Because 1) you are not making a good connection between the terminal and the battery post (Most probable if you know the battery is charged and the car started fine before you unhooked the battery.