The blinking green light tells you it has 120v. It should have been in alarm too. Plug the detectors back in and hold down (well, actually push up) on the reset button. It may take a moment. Repeat on the 2nd one. If they have 9v batteries and you don't know how old they are, replace them. Only 3? I've got 7 and when they go off, the whole neighborhood knows!
The amp rating is actually an amp/hour rating, it is a rating of capacity. A 4 amp/hour battery should be able to deliver 1 amp for 4 hours or 4 hours for 1 hour. A 7 amp/hour battery should deliver 1 amp for 7 hours or 7 amps for a hour. Make sure the physical dimensions will fit.