you need to reinstall the os. try command option R
that's internet recovery mode. if you have an ethernet cord plug that in and hit the power button, hold down those keys together and if it's an earlier macbook then it may not have internet recovery mode, and you may need to get an install disc, and pop it into the disc drive and boot the disc and reinstall the os that way... if you have no disc or the disc drive doesnt work or has issues... find an os.iso file somewhere online.. mac lets you download sierra os for free ive heard, although i had to fix my friends macbook pro through internet recovery mode myself, and its now mine... he gave it to me. pretty sweet deal!
anywho... when you get that iso file downloaded, put it on a flash drive big enough to hold the iso file or dmg. file whichever is... forgot which one i think its either or, maybe the iso... anywho...
if you dont have a usb flash drive, find an old external hard drive, maybe even old internal drive, format it, fat32, if you can format that drive with another mac and make sure its journaled +, and put the mac iso on it, so you can plug in into your macbook and boot it from that drive! you should be able to reinstall the os that way too!
so far, i've reinstalled mac os's on mac both through, external hard drives, as well as internet recovery mode which downloads mac os itself and reinstalls the os as long as your connected to the internet, and yes you can do internet recovery mode wirelessly too but it takes foreverrrrr, just plug your ethernet cable into it. keep in mind, internet recovery mode works only if its a newer ishhh mac... i used internet recovery mode on a 2011 macbook pro and reinstalled lion with that. would upgrade but im fine with lion myself with logic pro 9. anyway though... if you still can't get none of those options to work, try a little harder, make sure you've done everything correctly, don't over think it, slow down.. take it step by step, and if things still dont work, let's hear what's going on, again with more details.
thanks. hope that works!
peace