Dahlias like warm soil. They are not going to do any more growing until your soil warms up. They were sprouting in Walmart because it was warm and they sensed that it was time to grow. Usually people wait until near the last frost date to plant dahila tubers. Note that the tubers are not frost hardy so you will likely not be able to leave them in the ground next winter in your climate without special protection.

oh no. i bet they died in our frosts. :(

Oh yeah, I see you said you planted them back in January not right now. They probably froze and are now mush. You might as well dig them up and see for yourself what condition they are in. January is kind of early for Walmart to be selling dahlias but then again some people (like me) start them in pots indoors and then transplant them out around the last frost date to get a head start. The easiest and safest way to grow them is to plant the tuber outside after the last frost date but then you get flowers much later in the summer.