I had this problem on my MacBook Pro OS x Sierra.
I had to run Geth 1.7.3 from the terminal with this command
/Users/andrewhack/Downloads/geth-darwin/geth --fast --cache=2048
The directory will be different as it is where you download & unpack Geth 1.7.3
Once i had Geth running manually, i started up my Mist Wallet, it ignored Geth 1.7.2 the tick was not made when i checked. I had to do this for a couple of hours over a number of weeks. I did not let it run continuously.
My Mist Wallet eventually synced and i could see my Ethereum in my wallet, i did not delete my chaindata before running Geth 1.7.2 manually from terminal.
It is definitely a bug in Mist Wallet & hopefully that will get fixed in a update, so this is just a work around or temporary fix.
Have faith & stick with this method to the end, if this helps resolve your issue & get back your Ethereum that you thought was lost.
Consider donating a small percentage to disabled war veteran 0x9CBFf64255094129800c6Ea6d1f575e0C7B75DAf