how do you even know you had a 7700k to begin with? The major difference is the K is unlocked so you can overclock it. If you were having issues and you had increased the clock speed, wile troubleshooting its they likely set the clock speed back to default. So either you had a non K version, or you are judging it by what the clock speed is showing and they may have set it to default?

Follow up question, if you knew enough to know what exact cpu you had, why don't you do the work yourself next time? I'm sure with a little help from the internet you could have replaced the motherboard yourself.