SPFX web part issue running gulp serve

Hi, I have installed and was running the web parts by using commands, gulp serve in the past few days but now today after restarting my system when I run the command gulp serve in cmd, I'm getting 'gulp' is not recognized as an internal or external command. I do see not_modules folder and other folders but not sure why its not working now.

I do not want to reinstall npm gulp and want to know why this is happening if anyone experienced this.

Thanks,

-Sal