External Monitor Problems

I am using an external monitor with my MacBook Pro. It initially was working fine without any issues.

For some reason the MacBook is not recognizing it now.

Any recommendations?

What MBP? What version of macOS? What monitor? What have you done already to troubleshoot? What have you changed or installed since it was last working? You didn't give us much to go on, just "it doesn't work."

Nothing has changed running high sierra