Why would two regular outlets stop working after I replaced GFCI outlets?
There are a couple of possibilities for how this could happen, but the simplest theory is that you failed to reconnect the wire feeding those outlets to the LOAD side terminals on the GFCIs.  Turn the power off to the kitchen circuits and check in the GFCI-containing boxes for loose/unconnected wires.

If you don't find anything in there, there are three possibilities:


You knocked the wires feeding the outlets out of the boxes during the GFCI replacement.  Unfortunately, you'll probably need an electrician to come in and fix your divot for you in this case.
The outlets weren't GFCI protected, and you somehow managed to miss their HOT wire in the box -- perhaps your kitchen is on a multi-wire branch circuit, and the receptacle HOT wires never were connected back in when the GFCIs are replaced?
You interchanged the LOAD and LINE terminals when installing the new GFCIs -- the outlet itself will still function, but not as a GFCI, and it will not pass power "backwards".  What happens when you press the TEST button on the GFCI?  It should trip if and only if it's wired a semblance of correctly -- and will not trip if reverse-wired as there's no power on the LINE side for it to trip with.
The GFCI itself isn't passing power -- try tripping it manually with the TEST button then resetting it.  If everything is indeed wired correctly and it still doesn't pass power when tripped and reset, you have a pair of dud GFCIs on your hands, and it's time to replace 'em.