Short Answer
Uninstall Sype
Long Answer
Open Skype.
Tools -> Options -> Advanced -> Connection.
Disable the "Use port 80 and 443 for alternatives for incoming connections"
Sign Out and Close all Skype windows. Try restarting your Apache.
VERY LONG ANSWER
As I am working in a corporate environment where developers faces firewall issues, none of the other answers resolved my issue.
As the port is not used by Skype, but by some other internal applications, I followed the below steps to resolve the issue:
Step 1 - From the XAMPP Control Panel, under Apache, click the Config button, and select the Apache (httpd.conf).
Inside the httpd.conf file, somehow I found a line that says:
Listen 80
And change the 80 into any number / port you want. In my scenario I’m using port 8080.
Listen 8080
Still from the httpd.conf file, I found another line that says:
ServerName localhost:80
And change 80 to 8080.
ServerName localhost:8080
Step 2 - From the XAMPP Control Panel, under Apache, click the Config button again, but this time select the Apache (httpd-ssl.conf). Inside the httpd-ssl.conf file, find line that says
Listen 443
And change the 443 into any number / port you want. I’ll using 4433 as the new port number.
Listen 4433
Still from the httpd-ssl.conf file, find another line that says
ServerName localhost:443
And change 443 to 4433.
ServerName localhost:4433
Remember to save the httpd.conf and httpd-ssl.conf files after performing some changes. Then restart the Apache service.
SOURCE XAMPP, Apache - Error: Apache shutdown unexpectedly
CONCLUSION
Google first