Which model? You may have ended up without a working operating system, so you probably need to do it over again.
If you have a model with Internet Recovery capabilities then it's fairly easy to simply do a full reformat of the drive followed by a clean install of the operating system. You can enter it with option-command-R at power on to install the latest version of MacOS/OS X, or control-option-command-R to install the earliest version that Apple will support (which may or may not be the version that came with your Mac).