Came home last night and the a/c didn't work. The thermostat looked like it had no power running to it since the digital screen wasn't lit up. So this afternoon I bought a Honeywell rth2300 and now I'm lost. I hooked red to R green to G yellow to Y white to W and there's a blue one but I'm not sure where it goes. My sons and I are roasting and need help plzzz. I have a trane air handler and trane a/c unit outside

Most of the time the blue goes to common/C post,, but isn`t necessary if you use batteries in tstat,, the common /blue takes away the need for batteries in most cases

There also is a good chance you didn`t need a new tstat,, problem was somewhere else,, does old tstat need batteries? Did you replace them?, possible several other probs.